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Cell Motility and the Cytoskeleton,
1995]
The tubilin family has been considered to have two members, the a- and B-tubulins, which interact to form the heterodimers which in turn assemble to form the eukaryotic microtubules. A third member, y-tubulin, was identified in 1989 and has since been shown to be specifically localized in Microtubule Organizing Centers and has been implicated in the nucleation of microtubules in vivo. Comparisons of individual a-, B-, and y-tubulin sequences within the three subfamilies yield homologies of 65-100% identity. By contrast, comparisons between the three subfamilies typically yield homologies of only about 30-40% identity. The Caenorhabditis and yeast genome projects have recently identified two putative y-tubulin sequences. Analysis of these sequences, however, shows that they are significantly different from those of bona fide y-tubulins...
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Trends in Cell Biology,
1996]
Keeling and Logsdon propose that the y-like sequences from Caenorhabditis elegans and Saccharomyces cerevisiae are bona fide y-tubulins that have undergone rapid evolutionary divergence. Indeed, genetic and localization studies with the yeast epsilon-tubulin (encoded by the TUB4 gene) reveal striking similarities to the bona fide y-tubulins, whereas there is no apparent human analogue to the C. elegans delta-tubulin among the 60 available human y-tubulin expressed-sequence tags. (ESTs).
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Trends in Cell Biology,
1996]
Cellular microtubules assemble and disassemble at a variety of rates and frequencies, and these properties contribute directly to the cell-cycle-associated rearrangements of the microtubule cytoskeleton and to the molecular basis of mitosis. The kinetics of assembly/disassembly are governed, in part, by the hydrolysis of GTP bound to the B-tubulin nucleotide-binding site. The B-tubulin GTP-binding site, therefore, lies at the heart of microtubule assembly-disassembly kinetics, and the elucidation of its structure is central to an understanding of the cellular behaviour of microtubules. Unfortunately, the crystallographic structure of B-tubulin is not yet available. In this review, we describe the progress being made using mutagenesis and biochemical studies to understand the structure of this unusual GTP-binding site.