Sheetz, Michael

Michael Sheetz

Columbia University

Spudich, James

James Spudich

Stanford University School of Medicine

Vale, Ronald

Ronald Vale

University of California, San Francisco

For discoveries concerning cytoskeletal motor proteins, machines that move cargoes within cells, contract muscles, and enable cell movements.

The 2012 Albert Lasker Basic Medical Research Award honors three scientists for their discoveries concerning cytoskeletal motor proteins, machines that move cargoes within cells, contract muscles, and enable cell movements. By developing systems that allow reconstitution of motility from its constituent parts, Michael Sheetz (Columbia University), James Spudich (Stanford University School of Medicine), and Ronald Vale (University of California, San Francisco) established ways to study molecular motors in detail. These accomplishments enabled the discovery of the motor protein kinesin and unveiled the steps by which these engines convert chemical energy into mechanical work. The miniscule motors underlie numerous vital processes, and the landmark achievements of Vale, Spudich, and Sheetz are driving drug-discovery efforts aimed at cardiac problems as well as cancer.

Since 1774, when microscopist Bonaventura Corti discovered “torrents” of fluid inside plant cells, scientists have known that even tiny units of life bustle with motile activity. By the mid 1900s, researchers had observed chromosome separation during cell division and discovered that material travels long distances within nerve cells, but existing methods could not untangle these processes. Throughout most of the 20th century, the study of biological movement focused on muscle contraction, an activity that is fueled by the energy-rich molecule ATP.

In 1954, two British scientists, Hugh Huxley and Andrew Huxley (unrelated), independently proposed that muscles contract when filaments of two types — made of the proteins actin or myosin — slide past each other. Hugh Huxley later proposed that motion occurs when myosin molecules repeatedly grab actin, exert a stroke, and then let go. In this scenario, the mechanical force results from a precise set of structural changes in myosin when it binds ATP and breaks the high-energy bond. This model remained hard to test, however, because scientists lacked an experimental system that allowed them to piece together the reaction from its parts.

Fascinated by the mechanism with which ATP drives cellular activities, Spudich joined Hugh Huxley’s laboratory as a postdoctoral fellow. He was struck by the inability to reconstruct muscle contraction in a test tube. Spudich also learned that scientists had discovered actin and myosin inside nonmuscle cells; they had proposed that these proteins power intracellular movements, yet little was understood about the processes. When Spudich set up his own lab in 1971 (UCSF), he embarked on two quests: to establish a way to study movement in a test tube and to explore nonmuscle motility.

First test-tube system

Over the next dozen years, Spudich laid the groundwork for developing a test-tube — or in vitro — system. Among many challenges, he needed to find a source of actin filaments that faced the same way. Actin’s subunit proteins possess directionality and form fibers that ‘point’ in a certain direction. Inside cells, these tracks foster directed movement. Spudich positioned actin on glass slides and then added myosin-coated beads. Although some of the beads jiggled, they didn’t reliably trek along the filaments, presumably due to insufficient actin alignment.

In 1982, Sheetz (then at the University of Connecticut Health Center) began a sabbatical in Spudich’s lab, and the researchers decided to try working with the alga Nitella because it contained long and well oriented actin filaments. They slit the cells, splayed them open to expose the actin fibers, and added myosin-coated fluorescent beads. The beads streamed in a single direction along chloroplasts, which mark actin fibers. Spudich and Sheetz had, for the first time, created an in vitro assay for myosin movement. In 1983, they published a description of this system.

This innovation was momentous, but the Nitella cell innards were ill defined, and the work did not directly prove that myosin was moving on actin. Freshly invigorated, the researchers returned to the effort of building an actin scaffold on glass — much as nature did in the Nitella cells — on which they could assemble the minimal components for movement.

In 1985, Spudich, his student Stephen Kron, and Sheetz achieved this feat, creating an assay that allowed reconstitution of the process from scratch. They used the system to show that purified actin, purified myosin, and ATP could support myosin movement at rates consistent with the speed of muscle contraction.

A new motor protein

In the meantime, Vale had enrolled at Stanford as a neurobiology graduate student. The Sheetz and Spudich work captivated him: Perhaps, he thought, the actin-based system carried chemical-filled lipid bubbles, or vesicles, inside axons, the long protrusions of nerve cells.

In December 1982, a paper appeared that stoked Vale’s interest in this question. Using a video camera and computer hooked up to a microscope, Robert Allen, Scott Brady, and colleagues had peered inside the squid’s giant axon and witnessed an unexpected flurry of activity. Myriad particles — presumably organelles and vesicles — zipped through the cytoplasm. This behavior resembled that of myosin-coated beads in Nitella, so Sheetz and Vale decided to explore whether the particle movements in the squid’s giant axon relied on actin.

In 1983, they went to the Marine Biological Laboratory in Woods Hole and teamed up with microscopists Bruce Schnapp and Thomas Reese. The researchers harvested the innards of squid axons — including elements of the cytoplasm’s filamentous framework, or cytoskeleton. In the presence of ATP, organelles moved along tracks of unknown identity. When Schnapp and Reese turned the electron microscope on the samples, they found that the structure and composition of the filaments typified not actin, but microtubules — a different cytoskeletal protein. Much to the researchers’ surprise, they were not studying myosin-based movement.

Illustration of muscles

Now knowing that transport depended on microtubules, they could trade the mixed filament system in the squid for purified microtubules as they teased apart the reaction’s components. Reasoning that the organelle surfaces carry the engine that propels them, the researchers next added purified organelles and ATP to pure microtubules. Few organelles budged. When they added soluble proteins from the cytoplasm, the organelles whizzed along the fibers.

Additional experiments suggested that an abundant motor protein inhabits the cytoplasm — and that it fortuitously sticks to glass. When the investigators exposed glass to the soluble proteins and added microtubules, the glass-attached motors transported the filaments across the surface.

This new assay helped Vale and Sheetz purify the motor protein. In 1985, they named it kinesin, after the Greek root “kinein,” which means “to move.” Cow brains held a similar molecule, so it was not peculiar to squid. The team showed that kinesin differs from the only other protein known to bind microtubules and do work, dynein, suggesting that kinesin belongs to a novel class of force-generating molecules.

Like actin, microtubules possess directionality because their subunits attach in a head-to-tail fashion — and inside the cell, the microtubule filaments are arranged in a characteristic way. The investigators showed that kinesin movement corresponds to traffic from the center of a nerve cell to its axonal tips — and a different motor in the squid’s cytoplasm, which was subsequently identified as dynein, travels in the opposite direction.

In 1986, Kron and Spudich worked out a scheme — similar to the one developed for kinesin — for visualizing actin filaments gliding along myosin-coated surfaces. Together, this work from 1983-1986 established new and powerful ways to study movement in vitro, and these systems remain the gold-standard assays today.

Molecular motor mechanisms

With the assays for cytoskeletal motor activity in hand, researchers all over the globe began analyzing the molecules in detail. In 1987, Spudich discovered that myosin’s head constitutes that protein’s engine, an observation that focused scientists on that portion of the molecule. Vale established that an individual kinesin molecule can pull a microtubule and that this motor, unlike muscle myosin, can take many steps on its filament without detaching. Sheetz, Spudich, Vale, and others showed how myosin and kinesin harness ATP’s energy to generate the power strokes. For instance, Spudich measured the force and step size produced during one myosin reaction cycle, and Vale found that the parts of myosin and kinesin that bind ATP and drive movement strongly resemble each other structurally, even though their amino acid sequences are not similar.

Now we know that humans have dozens of kinesins and myosins. The motors differ in their cargoes and mechanistic details, but they share a strategy for converting small perturbations at the ATP-binding site into a force that provokes movement.

Motors in medicine

Myosin and kinesin operate in a broad range of physiological activities, and Spudich’s, Vale’s, and Sheetz’s breakthroughs have laid the foundation for many potential medical applications. Defects in genes for cardiac myosin can produce a disorder called hypertrophic cardiomyopathy, a leading cause of death in young athletes. Agents that might improve cardiac myosin performance are under investigation for this and other heart conditions. Flawed kinesin has been implicated in at least one neurological illness and, because kinesin is essential for cell division, which runs amok in cancer, chemotherapeutic drugs that target kinesins are being developed.

At every step, Vale, Sheetz, and Spudich devised robust experimental systems and ideas with which to attack stubborn problems. Through their vision, ingenuity, and persistence, these scientists opened the study of molecular motors and illuminated crucial features of a fundamental biological process.

by Evelyn Strauss

Key publications of Michael Sheetz

Sheetz, M.P. and Spudich, J.A. (1983). Movement of myosin-coated fluorescent beads on actin cables in vitro. Nature. 303, 31-35.

Vale, R.D., Reese, T.S., and Sheetz, M.P. (1985). Identification of a novel force-generating protein, kinesin, involved in microtubule-based motility. Cell. 42, 39-50.

Vale, R.D., Schnapp, B.J., Mitchison, T., Steuer, E., Reese, T.S., and Sheetz, M.P. (1985). Different axoplasmic proteins generate movement in opposite directions along microtubules in vitro. Cell. 43, 623-632.

Gelles, J., Schnapp, B.J., and Sheetz, M.P. (1988). Tracking kinesin-driven movements with nanometre-scale precision. Nature. 331, 450-453.

Kuo, S.C. and Sheetz, M.P. (1993). Force of single kinesin molecules measured with optical tweezers. Science. 260, 232-234.

Key publications of James Spudich

Sheetz, M.P. and Spudich, J.A. (1983). Movement of myosin-coated fluorescent beads on actin cables in vitro. Nature. 303, 31-35.

Spudich, J.A., Kron, S.J. and Sheetz, M.P. (1985). Movement of myosin-coated beads on oriented filaments reconstituted from purified actin. Nature. 315, 584-586.

Toyoshima, Y.Y., Kron, S.J., McNally, E.M., Niebling, K.R., Toyoshima, C., and Spudich, J.A. (1987). Myosin subfragment-1 is sufficient to move actin filaments in vitro. Nature. 328, 536-539.

Finer, J.T., Simmons, R.M., and Spudich, J.A. (1994). Single myosin molecule mechanics: Piconewton forces and nanometre steps. Nature. 368, 113-119.

Bryant, Z., Altman, D., and Spudich, J.A. (2006). The power stroke of myosin VI and the basis of reverse directionality. Proc. Natl. Acad. Sci. USA. 104, 772-777.

Key publications of Ronald Vale

Vale, R.D., Schnapp, B.J., Reese, T.S., and Sheetz, M.P. (1985). Organelle, bead, and microtubule translocations promoted by soluble factors form the squid giant axon. Cell. 40, 559-569.

Vale, R.D., Reese, T.S., and Sheetz, M.P. (1985). Identification of a novel force-generating protein, kinesin, involved in microtubule-based motility. Cell. 42, 39-50.

Howard, J., Hudspeth, A.J., and Vale, R.D. (1989). Movement of microtubules by single kinesin molecules. Nature. 342, 154-158.

Kull, F.J., Sablin, E.P., Lau, R., Fletterick, R.J., and Vale, R.D. (1996). Crystal structure of the kinesin motor domain reveals a structural similarity to myosin. Nature. 380, 550-555.

Rice, S., Lin, A.W., Safer, D., Hart, C.L., Naber, N., Carragher, B.O., Cain, S.M., Pechatnikova, E., Wilson-Kubalek, E.M., Whitaker, M., Pate, E., Cooke, R., Taylor, E. W., Milligan, R.A., and Vale, R.D. (1999). A structural change in the kinesin motor protein that drives motility. Nature. 402, 778-784.

Award presentation by Cori Bargmann

Award presentation by Cori BargmannIn the 12th century, religious monks of the Benedictine orders labored in the Scriptorium, where they copied and proofread religious texts and the manuscripts of antiquity. But other monks took the orders of “The Poor Fellow-Soldiers of Jesus Christ and the Temple of Solomon,” also called “the Knights Templars,” and these were men of action: soldiers in the Crusades and movers of trade, people, and resources throughout Europe and the Middle East.

The Lasker prize has recognized many basic science discoveries in the cellular equivalent of the Scriptorium. When we talk of DNA and RNA, we talk of replicating, transcribing, and translating the information of the genome — the copying and proofreading activities of the Benedictine monks. But this Lasker prize recognizes action: the mechanisms by which cells move, disseminate their subcellular components, and generate force. Our honorees, Jim Spudich, Mike Sheetz, and Ron Vale, showed how cells use the force-generating molecular motors myosin and kinesin to drive biological processes that include muscle contraction, the heartbeat, cell migration in development, cell division, and the orderly delivery of molecular components to each part of each cell.

The Lasker prize honors scientific discoveries that are important, universal, and beautiful. The studies of myosin and kinesin motor proteins done by Spudich, Sheetz and Vale are all three. The essence of their brilliant work was simplicity — taking biological processes that seemed very complex, and showing that at their heart they required just a few versatile and powerful proteins.

Before this work, it was known that specialized cells like muscles could generate force by contracting when a thick filament containing myosin marched along thin filaments made of actin. Spudich and Sheetz reduced this complex process to just two protein components. In a foundational experiment, they laid filaments made of actin down on a grid, tethering them at one end and combing them into alignment with fluid flow. They then added purified myosin and found that the myosin walked from one end of the actin track toward the other, consuming ATP as it went. The complex biological problem of muscle contraction can be reduced to a system with just two protein components: a polarized actin filament with distinguishable ends and an active myosin protein that moves in a directed fashion from one end to the other, converting the energy of ATP into work.

Another set of complex biological movements was reduced to simpler components through work at the Woods Hole Marine Biological Laboratory, where Ron Vale, then a graduate student at Stanford, and Mike Sheetz examined the movements of subcellular organelles. Cells are not just bags of stuff; they are precisely organized. If you want to wiggle your toes, you must send a nerve signal over a meter-long, micron-thin nerve process from your spinal cord to your foot, and that nerve process must then release acetylcholine only onto your foot muscles, and not anywhere along the way. This feat is only possible because the spinal cord motor neuron has first transported specific synaptic organelles down that meter-long nerve process, preparing the stage for rapid, local release. This essential process of directed axonal transport of vesicles and mitochondria was the subject of the Woods Hole group’s studies. Tom Reese and Bruce Schnapp, also at Woods Hole, found that axon organelles moved along a microtubule filament, not the actin filament as in contracting muscle. Sheetz and Vale then showed that the essential mediator of movement is a protein that binds to the microtubules and to the organelles that move along them and translocates itself along the microtubule while consuming ATP. They purified the translocating molecule, which they called “kinesin” for the Greek word for movement, and showed that this one molecule was the motor that marched along microtubules: When they fixed kinesin to glass, the microtubules slid along it in the presence of ATP. Again the movement was unidirectional, and again a complex cellular property could be reduced to just two protein elements.

The Lasker prize is for work that is not just important but universal. The discovery of motor proteins capable of directed migration on actin and microtubule filaments was important for its mechanistic description of how cells convert ATP into force and directed movement. It was also universal, because all cells contain structured actin filaments and microtubules, and Spudich, Sheetz, and Vale showed that all cells, not just specialized muscles and nerve cells, use myosin and kinesin motor proteins that move along the actin and microtubule cytoskeleton to organize their contents. Finally, the Lasker prize is for work that is beautiful, and these experiments were beautiful. Anyone who has watched microtubules glide along a kinesin-coated slide — an experiment that is still state-of-the-art today — is struck by the beauty of this molecular machine.

Watching the long movements of gliding microtubules led to a flood of insights into the biochemistry of cellular movements. Kinesin motors are processive — they don’t let go of the filament, but cling to it tenaciously. Like a child swinging on monkey bars, one hand is always attached while the other hand reaches for the next rung. And as a motor, kinesin is impressively fast, taking up to 100 steps along the microtubule in a second, and efficient, capturing about half of the energy released by each ATP that it hydrolyzes. There are families of kinesins and myosins, and each particular kinesin or myosin motor moves in a single direction along a filament, as though following a one-way street. Kinesins in particular are not just motors but transporters; different kinesins have different cargo-binding domains that bind specific cellular components, allowing them (for example) to specifically carry acetylcholine-containing vesicles along the axon’s microtubules to the synapse.

The versatility and universality of the motors comes from their ability to create completely different kinds of force in different situations. Imagine yourself with your hands on a rope, pulling one hand over the other. If the rope is fixed above you, you will climb up the rope. One the other hand, if the rope is loose on the ground, you will stand still and pull the rope toward yourself. In the same way, motor proteins can move themselves and their cargo through the cell on fixed actin or microtubule filaments, or they can pull loose filaments toward themselves, using the same basic hand-over-hand motion, and rearrange cellular components in a different way. All of these movements are elements of the most striking motor-driven process, the division of a cell into two daughters during mitosis. The orderly movements of chromosomes into the two nascent daughter cells, the rearrangements of microtubules in the spindle, and the pinching of one cell into two are all driven by kinesin and myosin motors.

Mike Sheetz, Jim Spudich, and Ron Vale illuminated these processes and many more. Their analysis of the motor proteins myosin and kinesin, and their elucidation of their biochemistry, biophysical properties, and structures, have led directly to our current understanding of the ways cells move themselves and their contents through active, dynamic, and specific interactions of motors with the actin and microtubule cytoskeleton.

Acceptance remarks

Interview with Michael Sheetz, James Spudich, and Ronald Vale

Video Credit: Susan Hadary