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Microtubule-Based Motility
Summary: Ron Vale studies the molecular motors that power the movement of membranes and chromosomes inside of cells as well as the role of the cytoskeleton in cell division and cell shape.
The ability of cells to move their internal contents has fascinated biologists for centuries. Our research focuses on motility generated by mechanochemical enzymes, termed motor proteins, that travel along microtubules, 25-nm-diameter polymers found in all eukaryotic cells. During interphase, the orchestrated efforts of microtubules and motors are required for organizing large intracellular membrane compartments and for transporting small membrane carrier vesicles in the endocytotic and secretory pathways. During mitosis, microtubules are the primary constituents of the mitotic spindle and are needed for proper segregation of chromosomes and for specifying the position of the cleavage furrow. Specialized arrays of microtubules are also used for ciliary and flagellar beating.
Although many microtubule motors have been identified in recent years, their precise biological roles and the mechanism by which they generate movement remain to be elucidated. Through investigations of the microtubule motors kinesin and dynein, we hope to develop a detailed understanding of the energy transduction pathway used by ATP-hydrolyzing protein motors. We are also interested in how molecular motors transport membrane organelles and how motors and numerous other cytoskeletal proteins govern complex cell behavior such as mitosis, migration, cell morphology, and signaling.
Elucidating the Force-Generating Mechanism
The biological machines that generate cell migration and intracellular movements consist of a protein motor that hydrolyzes ATP and a filamentous polymer (either actin filaments or microtubules) that provides a track for the unidirectional motion of the motor. We are studying biological movement generated by the microtubule motor kinesin. Kinesin's motor domain represents the smallest known molecular motor (~350 amino acids). Together with Robert Fletterick's laboratory (University of California, San Francisco), we determined the atomic resolution structure of the kinesin motor domain and discovered unexpectedly that it is similar in structure to myosin, an actin-based motor. We also use custom-built microscopes that can measure the force, steps, processivity, and velocity produced by single kinesin molecules.
Using a combination of single-molecule spectroscopy, cryoelectron microscopy (with Ron Milligan's group, Scripps Research Institute), pre-steady-state kinetics, and mutagenesis techniques, we identified a critical mechanical element in kinesin (called the neck linker) and showed that it undergoes nucleotide- and microtubule-dependent conformational changes. This allowed us to construct a structural model that explains the known features of kinesin movement. We also have shown recently that intramolecular tension conveyed through the neck linkers allows the two motor domains in the kinesin dimer to communicate with one another and efficiently couple ATP hydrolysis into forward stepping.
More recently, we have turned our attention to understanding the mechanism of dynein, the least understood of the cytoskeletal motor proteins. Dynein is a large molecule: the minimal motor domain is approximately 10-fold greater in size than kinesin's motor domain. Studying dynein has required strategies to express and mutagenize its large motor domain and to develop single-molecule assays to study its activity. We have developed budding yeast as a system for expressing and purifying the dynein motor. Using in vitro motility assays, we have observed single-molecule motility of cytoplasmic dynein and find that it is more processive than kinesin and generates a comparable amount of force. We also find that processivity (long-distance movement along a microtubule without detachment) requires two motor domains working in a coordinated manner. By attaching bright fluorescent markers (quantum dots), we also have clearly resolved individual steps taken by dynein motor domains. We have been able to coax dynein to take such steps by pulling on it with an optical trap without providing chemical energy (ATP). This result has led to a new model for dynein stepping. We are currently studying the structural changes that drive dynein motility, and we have recently obtained the crystal structure of dynein's microtubule-binding domain, in collaboration with Ian Gibbons (University of California, Berkeley).
Cytoskeleton and Membrane Organization
The cytoskeleton creates much of the asymmetry observed in nature. Controlling the dynamic assembly and disassembly of the cytoskeleton, therefore, is crucial for cell function. We have been using time-lapse imaging of living Caenorhabditis elegans to study cell migration and asymmetric cell division. We have found that asymmetric localization of myosin during cell division may contribute to the formation of different size daughter cells that have distinct cell fates.
We also have been using single-molecule and confocal microscopy to explore the organization of the plasma membrane. Using T cells as a model system, we have found that the plasma membrane is dotted with microdomains enriched in molecules that facilitate signal transduction pathways. These microdomains are held together by a dense network of protein-protein interactions, and not by phase separation of lipids into lipid raft domains. Recently, we have developed a live-cell FRET (fluorescence resonance energy transfer) sensor to measure T cell receptor phosphorylation and have shown that TCR phosphorylation persists in the endosome. Our next goal is to reconstitute the proximal steps of T cell signaling with T cell–specific signaling proteins introduced into nonimmune cells.
Identifying New Proteins Involved in Cytoskeletal Function
Recently, we have combined the gene-silencing strategy of RNAi (RNA interference) with high-resolution microscopy to identify roles of cytoskeletal proteins in determining cell shape and governing cell division. Using Drosophila S2 cells, we are performing whole-genome (14,000 genes) RNAi screens and using high-throughput microscopy to identify genes involved in cytoskeletal function. To better understand the roles of genes identified by such screens, we are tagging the proteins with green fluorescent protein (GFP) to determine their intracellular localization and dynamics. We are also performing live-cell imaging of the cytoskeleton after gene knockdown. We have just completed a whole-genome screen for mitotic spindle shape, which led to discoveries of several novel genes involved in mitosis, including genes that participate in microtubule formation, centriole assembly, chromosome alignment, spindle shape, and the spindle assembly checkpoint.
Currently, we are focusing our attention on two new proteins that emerged from the screen. Augmin is an eight-subunit complex that appears to be important for generating microtubules to form the mitotic spindle; we are currently trying to reconstitute its activity in vitro. Patronin has the long sought-after activity of binding to and protecting the microtubule minus end against depolymerization. We are also interested in the structural features of augmin and patronin that allow these proteins to execute their activities.
A grant from the National Institutes of Health provided support for the work on dynein.
As of July 29, 2010
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