Molecular Motors


Last Updated: January 10, 2008


Important Link: Stunning illustrations of these motors (and other features and processes within the cell) can be seen in the 8-minute animated video The Inner Life of a Cell, available on Studio Daily.


Hume vs. Paley: These "Motors" Settle the Debate, By Dr. Fazale Rana and Micah Lott


* Note: The sources quoted below attribute the details of these scientific discoveries to naturalistic evolution.

What is a "molecular motor"?

"Organisms, from human beings to bacteria, move to adapt to changes in their environments, navigating toward food and away from danger. Cells, themselves, are not static but are bustling assemblies of moving proteins, nucleic acids, and organelles (Figure 34.1). Remarkably, the fundamental biochemical mechanisms that produce contractions in our muscles are the same as those that propel organelles along defined paths inside cells. In fact, many of the proteins that play key roles in converting chemical energy in the form of ATP into kinetic energy, the energy of motion, are members of the same protein family, the P-loop NTPases. These molecular motors are homologous to proteins that we have encountered in other contexts, including the G proteins in protein synthesis, signaling, and other processes. Once again we see the economy of evolution in adapting an existing protein to perform new functions.

Molecular motors operate by small increments, converting changes in protein conformation into directed motion. Orderly motion across distances requires a track that steers the motion of the motor assembly. Indeed, we have previously encountered a class of molecular motors that utilize mechanisms that we will examine herenamely, the helicases that move along DNA and RNA tracks (Section 28.1.7). The proteins on which we will focus in this chapter move along actin and microtubules protein filaments composed of repeating identical subunits. The motor proteins cycle between forms having high or low affinity for the filament tracks in response to ATP binding and hydrolysis, enabling a bind, pull, and release mechanism that generates motion.

We will also consider a completely different strategy for generating motion, one used by bacteria such as E. coli. A set of flagella act as propellers, rotated by a motor in the bacterial cell membrane. This rotary motor is driven by a proton gradient across the membrane, rather than by ATP hydrolysis. The mechanism for coupling the proton gradient to rotatory motion is analogous to that used by the F0 subunit of ATP synthase (Section 18.4.2). Thus, both of the major modes for storing biochemical energy namely, ATP and ion gradients have been harnessed by evolution to drive organized molecular motion." (34. Molecular Motors)."

Molecular motors cooperate in moving cellular cargo, study shows

“CHAMPAIGN, Ill. — Researchers using an extremely fast and accurate imaging technique have shed light on the tiny movements of molecular motors that shuttle material within living cells. The motors cooperate in a delicate choreography of steps, rather than engaging in the brute-force tug of war many scientists had imagined.

“We discovered that two molecular motors – dynein and kinesin – do not compete for control, even though they want to move the same cargo in opposite directions,” said Paul Selvin, a professor of physics at the University of Illinois at Urbana-Champaign and corresponding author of a paper to appear in the journal Science, as part of the Science Express Web site, on April 7. “We also found that multiple motors can work in concert, producing more than 10 times the speed of individual motors measured outside the cell.”

Dynein and kinesin are biomolecular motors that haul cargo from one part of a cell to another. Dynein moves material from the cell membrane to the nucleus; kinesin moves material from the cell nucleus to the cell membrane. The little cargo transporters accomplish their task by stepping along filaments called microtubules.

To measure such minuscule motion, Selvin and colleagues at Illinois developed a technique called Fluorescence Imaging with One Nanometer Accuracy. The technique can locate a fluorescent dye to within 1.5 nanometers (one nanometer is a billionth of a meter, or about 10,000 times smaller than the width of a human hair). Recent improvements to FIONA now allow scientists to detect motion with millisecond time resolution.

Selvin’s team used FIONA to track fluorescently labeled peroxisomes (organelles that break down toxic substances) inside specially cultured fruit fly cells. This was the first time the imaging technique had been used inside a living cell.

“Our measurements show that both dynein and kinesin carry the peroxisomes in a step-by-step fashion, moving about 8 nanometers per step,” said Selvin, who also is a researcher at the Frederick Seitz Materials Research Laboratory on the Illinois campus.

“Because we see a fairly constant step size, we don’t believe a tug of war is occurring,” Selvin said. “If the dynein was fighting the kinesin, we would expect to see a lot of smaller steps as well.”

The researchers also noted that faster movements occurred with the same step size, but with greater rapidity. When measured outside the cell, kinesin moved about 0.5 microns per second. Inside the cell, the speed increased to 12 microns per second.

“There must be a mechanism that allows the peroxisomes to move by multiple motors much faster than independent, uncoupled kinesins and dyneins,” Selvin said. “It appears that motors are somehow regulated, being turned on or off in a fashion that prevents them from simultaneously dragging the peroxisome.”

In the future, Selvin wants to combine FIONA and an optical trap technique to monitor the speed and direction of a peroxisome, and the force acting upon it.

“By measuring force we can determine how many molecular motors are working together,” Selvin said. “This will help us further understand these marvelous little machines.”

Collaborators on the study included Illinois graduate students Comert Kural and Hwajin Kim (lead authors), Illinois professor of cell and structural biology Vladimir Gelfand (now at the Northwestern University School of Medicine) and postdoctoral research associates Sheyum Syed at Illinois and Gohta Goshima at the University of California at San Francisco.

The work was funded by the National Institutes of Health, the National Science Foundation, and the U.S. Department of Energy.

Editor’s note: To reach Paul Selvin, call 217-417-6101; e-mail:”


Clearing Jams in Copy Machinery

"Bacteria and humans use a number of tools to direct perhaps the most important function in cells -- the accurate copying of DNA during cell division. New research published this week in Molecular Cell from the laboratory of Rockefeller University's Michael O'Donnell, a Howard Hughes Medical Institute Investigator, now shows that one of these proteins, the beta sliding clamp, serves as a toolbelt from which the correct proteins are retrieved to enable DNA replication in the face of DNA damage.

The replication machinery inside the cell's nucleus is made up of a collection of enzymes including DNA polymerases, sliding clamps and clamp loaders. Bacteria have five known DNA polymerases (higher organisms such as humans have more). As the ring-shaped beta sliding clamp works its way along the DNA double helix, a network of proteins work together to unwind the two strands. Polymerases then add, in assembly line fashion, nucleotide bases -- the building blocks that make up DNA -- to convert the now-single-stranded templates into two new duplex DNA molecules.

The new research shows that two different DNA polymerases, the high fidelity Pol III replicase and the low fidelity Pol IV, coordinate their action to cross obstacles encountered in the replication process. They attach themselves at the same time to one beta sliding clamp. Pol III copies the original DNA, and acts as a proofreader to catch any misspellings and cuts any base that is wrong. But Pol III is a perfectionist, and can stall if it encounters a problem. Pol IV, on the other hand, lays down bases without checking for errors, keeping the process moving even when Pol III gets stuck. The findings by O'Donnell and his colleagues show that, because both polymerases are bound simultaneously to the beta clamp, it can pull either of the polymerases out if its toolbelt as needed.

O'Donnell and his colleagues propose two explanations for how the polymerase switch is controlled.

"One possibility is that the beta clamp may sense when Pol III stalls, triggering a change in beta that pulls the polymerase from the primed site, allowing Pol IV to take over synthesis," O'Donnell says. Or, Pol III, upon stalling, may loosen its grip on the template and allow Pol IV to bind the primed site instead.

Molecular Cell 19(6):805-814 (2005)


Functional coordination of intraflagellar transport motors

Nature 436, 583-587 (28 July 2005) | doi: 10.1038/nature03818

Functional coordination of intraflagellar transport motors
Guangshuo Ou1, Oliver E. Blacque2, Joshua J. Snow1, Michel R. Leroux2 and Jonathan M. Scholey1

"Top of pageCilia have diverse roles in motility and sensory reception, and defects in cilia function contribute to ciliary diseases such as Bardet−Biedl syndrome (BBS). Intraflagellar transport (IFT) motors assemble and maintain cilia by transporting ciliary precursors, bound to protein complexes called IFT particles, from the base of the cilium to their site of incorporation at the distal tip1, 2, 3. In Caenorhabditis elegans, this is accomplished by two IFT motors, kinesin-II and osmotic avoidance defective (OSM)-3 kinesin, which cooperate to form two sequential anterograde IFT pathways that build distinct parts of cilia4, 5, 6, 7. By observing the movement of fluorescent IFT motors and IFT particles along the cilia of numerous ciliary mutants, we identified three genes whose protein products mediate the functional coordination of these motors. The BBS proteins BBS-7 and BBS-8 are required to stabilize complexes of IFT particles containing both of the IFT motors, because IFT particles in bbs-7 and bbs-8 mutants break down into two subcomplexes, IFT-A and IFT-B, which are moved separately by kinesin-II and OSM-3 kinesin, respectively. A conserved ciliary protein, DYF-1, is specifically required for OSM-3 kinesin to dock onto and move IFT particles, because OSM-3 kinesin is inactive and intact IFT particles are moved by kinesin-II alone in dyf-1 mutants. These findings implicate BBS ciliary disease proteins and an OSM-3 kinesin activator in the formation of two IFT pathways that build functional cilia.

Top of page Center for Genetics and Development, Section of Molecular and Cellular Biology, University of California, Davis, California 95616, USA
Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
Correspondence to: Jonathan M. Scholey1 Correspondence and requests for materials should be addressed to J.M.S. (Email:

Received 16 April 2005; Accepted 9 May 2005"

Molecular engineering of a backwards-moving myosin motor

Nature 427, 558-561 (5 February 2004)

"Georgios Tsiavaliaris1,2, Setsuko Fujita-Becker2 and Dietmar J. Manstein1,2

Top of pageAll members of the diverse myosin superfamily have a highly conserved globular motor domain that contains the actin- and nucleotide-binding sites and produces force and movement1, 2. The light-chain-binding domain connects the motor domain to a variety of functionally specialized tail domains and amplifies small structural changes in the motor domain through rotation of a lever arm 3, 4. Myosins move on polarized actin filaments either forwards to the barbed (+ ) or backwards to the pointed (- ) end 5, 6. Here, we describe the engineering of an artificial backwards-moving myosin from three pre-existing molecular building blocks. These blocks are: a forward-moving class I myosin motor domain, a directional inverter formed by a four-helix bundle segment of human guanylate-binding protein-1 and an artificial lever arm formed by two -actinin repeats. Our results prove that reverse-direction movement of myosins can be achieved simply by rotating the direction of the lever arm 180°.

Top of pageInstitut für Biophysikalische Chemie, Medizinische Hochschule Hannover, OE 4350, Carl-Neuberg-Strasse 1, D-30623 Hannover, Germany
Abteilung Biophysik, Max-Planck-Institut für Medizinische Forschung, Jahnstrasse 29, D-69120 Heidelberg, Germany
Correspondence to: Dietmar J. Manstein1,2 Correspondence and requests for materials should be addressed to D.J.M. (Email:

Received 3 September 2003; Accepted 19 December 2003"

Marathon of Nano-Sprinters

November 14th, 2005

"Max Planck scientists show that the cooperation of a small number of molecular motors yields cargo transport over large distances

Processive bio-molecular motors, which move actively along cytoskeletal filaments, drive the cargo traffic in cells and in biomimetic systems. A single motor molecule is sufficient for continuous transport of cargoes such as vesicles or latex beads over a few micrometers. To achieve transport over larger distances, several motor mole-cules have to cooperate. Scientists from the Max Planck Institute of Colloids and In-terfaces in Potsdam have now developed a new theory that only seven or eight mo-tor molecules are sufficient for directed transport over centimeters or even meters. They also show that an applied load force, which is shared by the pulling motors, strongly reduces the cargo velocity and leads to a highly nonlinear force-velocity relationship (PNAS, Advanced Online Publication, November 14-18, 2005).

Image: Different bound states of a cargo particle, with several molecular motors which move along a filament. Each motor can unbind from and rebind to the filament, which implies that the number of motors that actually pull the cargo varies with time.

Image: Max Planck Institute of Colloids and Interfaces

Molecular motors are nano-tractors for all kinds of cargo within the cells of living beings. They move in a stepwise manner along filaments of the cytoskeleton, consuming energy provided by the hydrolysis of ATP, which can be considered the fuel of the cell. Kinesin and dynein motors move along microtubules and myosins move along actin filaments. The step sizes of these motors are of the order of 10 nm. By stepping in a directed fashion along filaments, the motors pull cargo particles which are much larger than the mo-tors themselves. In addition to their importance for the functioning of cells, molecular motors have many possible applications as biomimetic transport systems and are likely to become a key component in the emerging bio-nanotechnology.

Active transport driven by molecular motors is particularly important for nerve cells, or neurons. These cells have extended compartments, axons, which connect the cell body with the synapse, where the nerve signals are transmitted from one neuron to another. The length of axons is in the centimeter or even meter range; examples of relatively long axons are those that connect our spinal cord with the tips of our fingers and toes. Within such an axon, microtubules provide the tracks along which molecular motors transport their cargo, such as vesicles filled with neurotransmitters.

During the last decade, our knowledge about molecular motors has increased rapidly. This was mainly due to the development of powerful single molecule experiments and biomimetic model systems which permit the study of molecular motors outside cells in a systematic fashion. One example is the bead assay, where filaments are immobilized on a surface. Molecular motors pull latex beads along these filaments, and the movement of the beads is observed under the microscope.

One important result of these experiments is that molecular motors, unlike railways or cars, have a strong tendency to fall off their track and diffuse away into the surrounding aqueous solution. This is a direct consequence of their nanoscale size which makes them rather susceptible to thermal noise. Thus, a single molecular motor can only ’grab’ onto the filament for a relatively short time, on the order of one second. During this time, a single motor covers about one micrometer, which represents only a tiny fraction, about 1/10000, of the long transport distances for cargo particles in axons. In other words, a single motor behaves much like a sprinter, whereas the whole cargo performs a marathon.

Scientists of the Max Planck Institute of Colloids and Interfaces in Potsdam have now provided a simple solution to this puzzle. If the cargo is pulled by several motors as shown in Fig.1, any motor that unbinds from the filament will stay close to that filament as long as the cargo and filament are still cross-linked by at least one bound motor. In such a situation, the unbound motor can rebind to the filament and then continue to pull the cargo - in contrast to human sprinters, molecular motors don’t get tired.

This mechanism has been derived from a new theoretical model, which distinguishes the different bound states of the cargo particle and describes the transitions between these states. Using this model, the Max Planck scientists have been able to calculate several transport properties, such as the average velocity and the average walking distance of the cargo particle as a function of the maximum number of motors that can pull this par-ticle. For kinesin motors, for example, calculations show that only seven or eight motors are sufficient for the transport over centimeter distances and that a cargo particle pulled by 10 motors has an average walking distance of about 1 meter.

If molecular motors move against an external load force, this force is shared among the pulling motors. One obvious consequence is that the movement of the cargo is slowed down. In addition, the force felt by each pulling motor strongly increases the unbinding probability for such a motor. Furthermore, as more motors unbind, each of the remaining pulling motors has to sustain a larger force, which would mean that their unbinding probability increases even further. This leads to a cascade of unbinding processes and to a strongly nonlinear dependence of the cargo velocity on the external load force. Simi-lar cascade processes are expected in more complex situations, in which the cargo transport is performed by different types of molecular motors.

All transport properties predicted by the new theory can be investigated in experiments using techniques which have been developed for single motors. In fact, preliminary ex-periments at the Max Planck Institute in Potsdam agree with the theoretical predictions. Likewise, the quantitative theory should also be useful in order to design biomimetic transport systems for lab-on-a-chip applications -- in which, for example, molecular mo-tors transport certain molecules to specific reaction sites. Depending on the arrange-ment of the filaments in these systems, varying the travel distance provides a strategy to control either the localization of the reagents to their target sites or, alternatively, their diffusion, which is enhanced by motor-driven active transport."


Purdue biologists clarify how a cellular 'spacecraft' opens its airlock

December 2, 2005

"WEST LAFAYETTE, Ind. – Scientists have a tough time visualizing the tiny hatchways that allow nutrients to pass into our cells, but a group of Purdue University biologists may have found the next best thing: a glimpse into the workings of the "motor" that opens and closes them.

A research team led by Jue Chen has clarified the connection between these minuscule gates – which are called membrane transport proteins – and the steps by which they use a cell's energy to permit or deny materials entry into the interior of the cell from the outside world.

In what the team perceives to be a three-step process, cells feed chemical energy to a tiny machine called an ABC protein, which is the part of the membrane protein that connects it to the interior of the cell. These ABC proteins use the energy to bend the membrane protein into its open and closed positions, allowing the cell both to bring in nutrients and to flush out waste.

"We think we have a better handle on a process fundamental to life in creatures from bacteria to humans," said Chen, who is an assistant professor of biology in Purdue's College of Science. "This is the first time the entire cycle has been visualized, and this could enhance our understanding of how the process of metabolism unfolds."

The team's paper appears in this week's issue of Proceedings of the National Academy of Sciences. Chen's group also includes her Purdue colleagues Gang Lu and James M. Westbrooks, as well as Amy L. Davidson, who recently relocated to Purdue from the Baylor College of Medicine. The team used X-ray crystallography and other advanced imaging techniques to obtain a clear picture of the ABC protein, a method which has only had limited success in revealing secrets of the membrane proteins themselves.

Membrane proteins in cells have been likened to spacecraft airlocks, which ensure that only the astronauts gain entry and no air is lost. Where spacecraft have metal walls, cells have membranes that surround their inner protoplasm, and their airlock proteins are highly complex individual molecules that allow nutrients to enter cells and waste products to leave them.

Of the thousands of membrane proteins that exist, scientists only know the structure of a few dozen. They are of great interest to biologists because, as the regulators of intercellular commerce, they essentially permit metabolism – and, thus, life itself – to continue. However, while most proteins dissolve in water and can be easily crystallized and examined, membrane proteins dissolve only in fatty substances, making it difficult to isolate them for study.

"If we had a better understanding of this class of proteins, we might know more about how our bodies use and transfer energy," Chen said. "It's an unfortunate gap in our knowledge of how living things work. But in this study, we looked at a protein that is a bit of a hybrid: one part of it is fat-soluble, and the other is water-soluble."

Because the entire membrane protein would not submit to crystallization, Chen's team focused their efforts on the ATP-binding cassette proteins, or ABC proteins for short, that connect the membrane proteins with the cell's interior. This portion of the protein is of the more study-friendly, water-soluble variety, and also plays a critical role in cellular commerce: It is the motor that drives a membrane protein's motion.

"We isolated the ABC proteins from an E. coli bacterium, which is a very common research subject," Chen said. "Different as these single-celled organisms are, their ABC proteins are structurally very similar to those in human cells, so studying them could help our knowledge of our own metabolism."

ABC proteins function like tiny tweezers and are powered by ATP, a chemical that animal cells use for energy. When ATP causes the tweezers to squeeze shut, the membrane proteins open to reveal a small cavity that can hold a nutrient or other substance the cell requires from the outside. Once the nutrient is in place, the cell uses water to break down the ATP, signaling the "tweezers" to relax, closing the membrane protein gate and capturing the nutrient. Lastly, the membrane protein releases the nutrient into the cell's interior.

"The ABC protein is like the inner door of the airlock; that's what we were able to see in operation in this study," Chen said. "If you opened both it and the membrane protein simultaneously, nothing would stop the interior of the cell from getting sucked out."

Chen admits that the team is not yet certain that the description of the process is complete, though it does seem compelling based on what science already knows about the workings of membrane proteins.

"We need to look closer at our information and try to find out more," Davidson said. "We will be applying several tests to our data in the near future to determine if our image of these proteins accurately describes their behavior."

Chen said the work might have long-term payoffs in the fight against cancer, though it was too soon to make more than general statements as to how.

"Many cancer cells are resistant to anticancer drugs because the ABC proteins are overabundant and get too good at pumping the drugs out before they can work," she said. "Future therapies might exploit what we are finding out about these proteins' operation. It's too soon to talk about specific therapies, but because there are so many kinds of cancer out there, every piece of knowledge helps."

This research was sponsored in part by the National Institutes of Health and the Pew Charitable Trusts.

Members of Chen's research group are associated with the Purdue Cancer Center. One of just seven National Cancer Institute-designated basic-research facilities in the United States, the center attempts to help cancer patients by identifying new molecular targets and designing future agents and drugs for effectively detecting and treating cancer. The Cancer Center is part of the Oncological Sciences Center in Purdue's Discovery Park.

Writer: Chad Boutin, (765) 494-2081,

Sources: Jue Chen, (765) 496-3113,

Amy Davidson, (765) 494-5291,

Purdue News Service: (765) 494-2096;"



Direct observation of steps in rotation of the bacterial flagellar motor

"Nature 437, 916-919 (6 October 2005) | doi: 10.1038/nature04003
Direct observation of steps in rotation of the bacterial flagellar motor

Yoshiyuki Sowa1,5, Alexander D. Rowe2,5, Mark C. Leake2, Toshiharu Yakushi3, Michio Homma3, Akihiko Ishijima1,4 and Richard M. Berry2
Top of page

The bacterial flagellar motor is a rotary molecular machine that rotates the helical filaments that propel many species of swimming bacteria1, 2. The rotor is a set of rings up to 45 nm in diameter in the cytoplasmic membrane3; the stator contains about ten torque-generating units anchored to the cell wall at the perimeter of the rotor4, 5. The free-energy source for the motor is an inward-directed electrochemical gradient of ions across the cytoplasmic membrane, the protonmotive force or sodium-motive force for H+-driven and Na+-driven motors, respectively. Here we demonstrate a stepping motion of a Na+-driven chimaeric flagellar motor in Escherichia coli6 at low sodium-motive force and with controlled expression of a small number of torque-generating units. We observe 26 steps per revolution, which is consistent with the periodicity of the ring of FliG protein, the proposed site of torque generation on the rotor7, 8. Backwards steps despite the absence of the flagellar switching protein CheY indicate a small change in free energy per step, similar to that of a single ion transit.

Top of page

1. Department of Applied Physics, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
2. The Clarendon Laboratory, Department of Physics, Oxford University, Parks Road, Oxford OX1 3PU, UK
3. Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
4. PRESTO, Japan Science and Technology Corporation (JST) 4-1-8, Honmachi, Kawagoe, Saitama 332-0012, Japan
5. *These authors contributed equally to this work

Correspondence to: Richard M. Berry2 Correspondence and requests for materials should be addressed to R.M.B. (Email:

Received 1 April 2005; Accepted 11 July 2005"


Cooperative cargo transport by several molecular motors

( active transport | bionanosystems | load force | run length | walking distance )
Stefan Klumpp and Reinhard Lipowsky *

Max Planck Institute of Colloids and Interfaces, Science Park Golm, 14424 Potsdam, Germany

Edited by David R. Nelson, Harvard University, Cambridge, MA, and approved October 3, 2005 (received for review August 26, 2005)

"The transport of cargo particles that are pulled by several molecular motors in a cooperative manner is studied theoretically in this article. The transport properties depend primarily on the maximal number N of motor molecules that may pull simultaneously on the cargo particle. Because each motor must unbind from the filament after a finite number of steps but can also rebind to it again, the actual number of pulling motors is not constant but varies with time between zero and N. An increase in the maximal number N leads to a strong increase of the average walking distance (or run length) of the cargo particle. If the cargo is pulled by up to N kinesin motors, for example, the walking distance is estimated to be 5N-1/N micrometers, which implies that seven or eight kinesin molecules are sufficient to attain an average walking distance in the centimeter range. If the cargo particle is pulled against an external load force, this force is shared between the motors, which provides a nontrivial motor-motor coupling and a generic mechanism for nonlinear force-velocity relationships. With increasing load force, the probability distribution of the instantaneous velocity is shifted toward smaller values, becomes broader, and develops several peaks. Our theory is consistent with available experimental data and makes quantitative predictions that are accessible to systematic in vitro experiments.

Author contributions: S.K. and R.L. designed research, performed research, and wrote the paper.

Conflict of interest statement: No conflicts declared.

Freely available online through the PNAS open access option.

*To whom correspondence should be addressed.
Reinhard Lipowsky, E-mail:"


Biological motors sort molecules one by one on a chip
12 May 2006 by M&C

"Researchers from Delft University of Technology’s Kavli Institute of Nanoscience have discovered how to use the motors of biological cells in extremely small channels on a chip. Based on this, they built a transport system that uses electrical charges to direct the molecules individually. To demonstrate this, the Delft researchers sorted the individual molecules according to their color. Professor Hess of the University of Florida has called the Delft discovery "the first traffic control system in biomolecular motor nanotechnology". The research findings will be published in Science on May 12.

The biological cell is a complex of many different small protein factories. The necessary transportation of materials within the cell occurs across a network of microtubules: long, tubular-shaped proteins that extend in a star-shaped formation from the nucleus of the cell to the walls of the cell. Molecular bio-motors, such as the enzyme kinesin, can walk in small steps (of 8 nanometers) with a load of material along these microtubule-networks and thus provide transport within the cell.

Fascinated by these biological motors, the researchers at Delft University of Technology’s Kavli Institute of Nanoscience are currently exploring the possibility of inserting these kinesin-motors and microtubules in an electrically directed transport system that is made by the researchers using nano-fabrication techniques.
The researchers turned the system around: the kinesin-motors are fastened in large quantities on a surface with their 'feet' up; the microtubules (measuring approximately 1 to 15 micrometers in length) were then transported over the 'carpet' of motors. The microtubules are, as it were, 'crowd surfing' over the sea of small kinesin motors. A particular challenge of the research was to ensure beforehand that the microtubule tubes could be transported in a determined direction and were not dislodged by collisions of the motor carpet.

PhD student Martin van den Heuvel, master student Martijn de Graaff and groupleader Professor Cees Dekker have for the first time achieved to control and address individual microtubules. An important step in this was to allow microtubule-transport to occur in small closed liquid channels. This made it possible to apply a strong electrical field locally at the Y-junction in the channels. Because of this, the electrical force could be exerted on the individual microtubules. The researchers discovered that by using this electrical force they could push the front of the microtubule into the determined direction.

To demonstrate this, the researchers allowed a mixture of green and red fluorescent microtubules to arrive at a Y-junction. By changing the direction of the electrical force, depending on the color of the microtubule, the Delft researchers were able to collect the green and red microtubules in different reservoirs.

With their approach to the nano-channels, the researchers killed two birds with one stone. In addition to the possibility of steering individual microtubules, they were able to prevent the microtubules from derailing from their tracks. Incidentally, the Delft researchers discovered that their work contained a third interesting aspect. The closed channels offered the possibility to observe the electrical transport of freely suspended microtubules, thus proving that the speed of the microtubules under an external electrical field is largely dependent on the orientation of the cylinder-shaped molecules. This was the first time that this orientation-dependency of the electrophoretic mobility was observed.

In an accompanying Perspective article in Science, Professor Hess of the University of Florida wrote that the Delft researchers had developed the first traffic control system in biomolecular motor nanotechnology.

For additional information:

* Cees Dekker
T: +31 15 278 6094,
* Frank Nuijens, science information officer,
T: +31 15 278 4259,


Myosin-V is a mechanical ratchet

"( backward movement | molecular motor | optical tweezers | asymmetry | kinesin )
J. Christof M. Gebhardt, Anabel E.-M. Clemen, Johann Jaud, and Matthias Rief *

Physics Department E22, Technical University of Munich, James-Franck-Strasse, D-85748 Garching, Germany

Edited by Ronald D. Vale, University of California, San Francisco, CA, and approved April 6, 2006 (received for review November 24, 2005)

Myosin-V is a linear molecular motor that hydrolyzes ATP to move processively toward the plus end of actin filaments. Motion of this motor under low forces has been studied recently in various single-molecule assays. In this paper we show that myosin-V reacts to high forces as a mechanical ratchet. High backward loads can induce rapid and processive backward steps along the actin filament. This motion is completely independent of ATP binding and hydrolysis. In contrast, forward forces cannot induce ATP-independent forward steps. We can explain this pronounced mechanical asymmetry by a model in which the strength of actin binding of a motor head is modulated by the lever arm conformation. Knowledge of the complete force-velocity dependence of molecular motors is important to understand their function in the cellular environment.

Author contributions: M.R., A.E.-M.C., and J.J. designed research; J.C.M.G. performed research; J.C.M.G. analyzed data; and M.R. and J.C.M.G. wrote the paper.

Conflict of interest statement: No conflicts declared.

*To whom correspondence should be addressed.
Matthias Rief, E-mail:"


On the hand-over-hand mechanism of kinesin

"Qiang Shao, and Yi Qin Gao

Department of Chemistry, Texas A&M University, College Station, TX 77843

Communicated by Rudolph A. Marcus, California Institute of Technology, Pasadena, CA, April 6, 2006 (received for review December 8, 2005)

We present here a simple theoretical model for conventional kinesin. The model reproduces the hand-over-hand mechanism for kinesin walking to the plus end of a microtubule. A large hindering force induces kinesin to walk slowly to the minus end, again by a hand-over-hand mechanism. Good agreement is obtained between the calculated and experimental results on the external force dependence of the walking speed, the forward/backward step ratio, and dwell times for both forward and backward steps. The model predicts that both forward and backward motions of kinesin take place at the same chemical state of the motor heads, with the front head being occupied by an ATP (or ADP,Pi) and the rear being occupied by an ADP. The direction of motion is a result of the competition between the power stroke produced by the front head and the external load. The other predictions include the external force dependence of the chemomechanical coupling ratio (e.g., the stepping distance/ATP ratio) and the walking speed of kinesin at force ranges that have not been tested by experiments. The model predicts that the chemomechanical coupling remains tight in a large force range. However, when the external force is very large (e.g., {approx}18 pN), kinesin slides in an inchworm fashion, and the translocation of kinesin becomes loosely coupled to ATP turnovers."


Researchers probe the machinery of cellular protein factories

September 13, 2006

Contact: Tim Stephens (831) 459-2495;

"Proteins of all sizes and shapes do most of the work in living cells, and the DNA sequences in genes spell out the instructions for making those proteins. The crucial job of reading the genetic instructions and synthesizing the specified proteins is carried out by ribosomes, tiny protein factories humming away inside the cells of all living things.

Harry Noller, the Sinsheimer Professor of Molecular Biology at the University of California, Santa Cruz, has been studying the ribosome for more than 30 years. His main goal is to understand how the ribosome works and how it evolved, but there are also practical reasons to pursue this research. Many of the most effective antibiotics work by targeting bacterial ribosomes, and findings by Noller and others have led to the development of novel antibiotics that hold promise for use against germs that have developed resistance to current drugs. Drug-resistant staph infections, for example, are a serious problem in hospitals.

Noller's laboratory achieved breakthroughs in 1999 and 2001, producing the first high-resolution images of the molecular structure of a complete ribosome. Now, his team has made another major advance with an even higher-resolution image that enables them to construct an atom-by-atom model of the ribosome.

The new picture shows details never seen before and suggests how certain parts of the ribosome move during protein synthesis. A paper describing the new findings will be published in the September 22 issue of the journal Cell and is currently available online.

"We can now explain a lot of the results from biochemical and genetic studies carried out over the past several decades," Noller said. "This structure gives us another frame in the movie that will eventually show us the whole process of the ribosome in action."

The ribosome is a complex molecular machine made up of proteins and RNA molecules. The bacterial ribosomes studied in Noller's lab (obtained from the bacterium Thermus thermophilus) are made up of three different RNA molecules and more than 50 different proteins.

Noller proposed in the early 1970s that the RNA component was responsible for carrying out the ribosome's key functions. At the time it was considered a "crackpot idea," but subsequent findings by Noller and others proved he was right.

"It was a completely heterodox view when we first proposed it, but it is now the accepted paradigm," said Noller, who directs the Center for Molecular Biology of RNA at UCSC. "Our latest results confirm that the ribosomal RNA is really the key to ribosome function. The proteins are also involved, but more peripherally," he said.

To make a new protein, the genetic instructions are first copied from the DNA sequence of the gene into a messenger RNA molecule. The ribosome then reads the genetic code from the messenger RNA and translates it into the structure of a protein.

Proteins are linear molecules that fold into complex three-dimensional shapes to carry out their functions. They are made from amino acid building blocks, and the sequence of amino acids determines the protein's structure. Amino acids are carried to the ribosome by transfer RNA molecules. On the ribosome, the transfer RNAs recognize specific sequences of genetic code on the messenger RNA, and the amino acids are then joined together in the proper order.

The images from Noller's group not only show the complete ribosome, they show it with a messenger RNA and two full-length transfer RNAs bound to it. "We can now see the details of most of the interactions between the ribosome, the messenger RNA, and the transfer RNAs," Noller said.

The results provide a snapshot of the molecular machine in action. By comparing his images with those obtained by other groups that have caught the ribosome or its subunits in different positions, Noller is finding clues to the molecular motions with which the ribosome does its work.

"Our next goal is to trap the ribosome in other functional states to get more frames of the movie," he said.

The authors of the paper, in addition to Noller, are postdoctoral researcher Andrei Korostelev, senior scientist Sergei Trakhanov, and postdoctoral researcher Martin Laurberg. The researchers used a technique called x-ray crystallography, which involves growing crystals of purified ribosomes, shining a focused beam of x-rays through the crystals, and analyzing the resulting diffraction pattern. Trakhanov prepared the crystals and Korostelev and Laurberg performed the crystallography and solved the structure, Noller said.

This research was supported by grants from the National Institutes of Health and the Agouron Institute."


Molecular Motors and Brakes Work Together in Cells

January 30, 2007

"Interaction sheds light on how cells inner skeleton is organized

Researchers at the University of Pennsylvania School of Medicine have discovered that microtubules – components responsible for shape, movement, and replication within cells – use proteins that act as molecular motors and brakes to organize into their correct structure. If microtubules are not formed properly such basic functions as cell division and transport can go wrong, which may have implications in such disease processes as cancer and dementia. The study, published in the January issue of Cell, is featured on the cover of that issue.

“Up until now motors and brakes were studied separately from microtubules,” says senior author Phong Tran, PhD, Assistant Professor of Cell and Developmental Biology. “This study lets us have a more complete picture.”

Microtubules are structures that help give shape to many types of cells, form the spindle – a structure important in cell division – and act as a railroad, of sorts, upon which molecular motors move protein packages for waste removal and nerve transmission.

In the Cell study, the investigators, working with fission yeast cells, showed that stable end-to-end arrays of microtubules can be achieved by a balance between the sliding by a molecular motor called klp2p and the braking of a microtubule-associated protein (MAP) called ase1p. Specifically, they showed that a preexisting “mother” microtubule acts as a platform on which a new microtubule can be formed. The new “daughter” microtubule grows and moves along the mother microtubule. In time, the daughter grows beyond the end of the mother to ultimately produce two microtubules, connected by the cross-linking MAP ase1p.

“Imagine that the daughter microtubule is a short train on the track of the mother microtubule,” explains Tran. “The molecular motor is the train’s engine, but the problem is that the cargo – the molecular brakes – gets longer, slowing down the daughter train. But when the train gets to the end of the track it remains attached to the end of mother microtubule. At the tail end, it stops moving and that defines the region of overlap. Our work shows that the cell can make microtubule structures of defined lengths stable by coordinating the sliding of the motors and the slowing of the brakes.”

If microtubule-based structures are not formed properly because of failures in brakes or motors, such basic functions as cell division and cell transport can go awry, with such diseases as cancer and dementia possibly resulting. “For the first time we have shown how MAPs and motors work together in a mechanistic way,” says Tran. “This is important and it will make other people who study microtubules rethink how they study the cell.”




What is a Molecular Motor?

Molecular motors cooperate in moving cellular cargo, study shows

Clearing Jams in Copy Machinery

Functional coordination of intraflagellar transport motors

Molecular engineering of a backwards-moving myosin motor

Marathon of Nano-Sprinters

Purdue biologists clarify how a cellular 'spacecraft' opens its airlock

Direct observation of steps in rotation of the bacterial flagellar motor

Cooperative cargo transport by several molecular motors

Biological motors sort molecules one by one on a chip

Myosin-V is a mechanical ratchet

On the hand-over-hand mechanism of kinesin

Researchers probe the machinery of cellular protein factories

Molecular Motors and Brakes Work Together in Cells




























































































































































































































































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