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- Worms in space |
Skeletons without bones; motors without wires; eukaryotic cells have them and more. The wide variety of cell shapes are largely a function of a collection of fibrous structural proteins called the cytoskeleton. Made up of microfilaments, intermediate filaments and microtubules, the cytoskeleton not only determines a cells shape but also its motility. Cells move themselves, they move the surrounding medium, and they move organelles within the cytoplasm. These movements require the cytoskeleton as a structural matrix and motor proteins to provide the force. Motor proteins promote sliding between cytoskeletal elements and they carry cargo in the form of vesicles or organelles along the cytoskeleton.
Susan Gilbert's laboratory is engaged in detailed biochemical and kinetic analyses of the microtubule based molecular motors, Ncd and kinesin. Conventional kinesin is a neuronal ATPase that drives membranous organelle movements toward the synapse. In contrast, Drosophila Ncd is an essential motor for chromosome segregation during female meiosis and the early cleavages of the embryo. Ncd, unlike kinesin, moves one microtubule relative to another for the organization and maintenance of the spindle. Although the motor domains of kinesin and Ncd are highly conserved both in 3-D crystal structure and amino acid sequence, Ncd is a minus-end directed motor while kinesin is a plus-end directed motor. Thus, kinesin and Ncd are model systems to define the mechanism that specifies the direction of movement along a microtubule, the unique characteristics of ATP turnover that result in very different cellular movements functionally, and the structural domains that generate the distinctive behavior of the kinesin and Ncd ATPases.
Bill Saunders' laboratory is defining the role of the multiple molecular motors needed for the formation and function of the mitotic spindle using the power of genetic analysis in the yeast, Saccharomyces cerevisiae. By combining mutagenesis with advanced techniques in fluorescence microscopy (see Fig. 1 above), the Saunders group has identified new components and new functions for motor proteins in the mitotic spindle. These studies have characterized Cin8p and Kip1p motors related to the vesicle transporting motor, kinesin as well as Dyn1p thought to be the yeast homologue of vertebrate cytoplasmic dynein. A second, related area of interest in the Saunders lab is why mitosis occurs only once and at a specific time in the cell cycle. The mitotic motors are also excellent candidates for genes that may become defective in cancer cells. The lab plans to determine if defects in the human homologs of the genes encoding these proteins are correlated with tumor formation.
Charles Walsh's laboratory is investigating the processes responsible for the synthesis and assembly of the microtubule cytoskeleton and flagella in the amebo-flagellate Naegleria gruberi using techniques of molecular biology and light and electron microscopy. Naegleria amebae have the unique property of being able to transiently differentiate into a swimming flagellated form. In the process the cell change from an actin-myosin based motility system to one driven by the molecular motor dynein in the flagella. Current work is focused on the defining the mechanism by which the unique structure of basal bodies, containing nine sets of triplet microtubules, is assembled in just 50 min during the differentiation. Basal bodies share the nine triplet structure with centrioles, structures found in eukaryotic cells from protozoa through mammals. |
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