Tubulin sorting during dimerization in vivo.
We demonstrate sorting of beta-tubulins during dimerization in the Drosophila male germ line. Different beta-tubulin isoforms exhibit distinct affinities for alpha-tubulin during dimerization. Our data suggest that differences in dimerization properties are important in determining isoform-specific microtubule functions. The differential use of beta-tubulin during dimerization reveals structural parameters of the tubulin heterodimer not discernible in the resolved three-dimensional structure. We show that the variable beta-tubulin carboxyl terminus, a surface feature in the heterodimer and in microtubules, and which is disordered in the crystallographic structure, is of key importance in forming a stable alpha-beta heterodimer. If the availability of alpha-tubulin is limiting, alpha-beta dimers preferentially incorporate intact beta-tubulins rather than a beta-tubulin missing the carboxyl terminus (beta 2 Delta C). When alpha-tubulin is not limiting, beta 2 Delta C forms stable alpha-beta heterodimers. Once dimers are formed, no further sorting occurs during microtubule assembly: alpha-beta 2 Delta C dimers are incorporated into axonemes in proportion to their contribution to the total dimer pool. Co-incorporation of beta 2 Delta C and wild-type beta 2-tubulin results in nonmotile axonemes because of a disruption of the periodicity of nontubulin axonemal elements. Our data show that the beta-tubulin carboxyl terminus has two distinct roles: 1) forming the alpha-beta heterodimer, important for all microtubules and 2) providing contacts for nontubulin components required for specific microtubule structures, such as axonemes.
Hoyle
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Department of Biology and Indiana Molecular Biology Institute, Indiana University, Bloomington, Indiana 47405, USA. hhoyle@bio.indiana.edu
Turner
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Brunick
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Raff
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D000818:Animals; D019281:Dimerization; D004331:Drosophila melanogaster / Q000378:metabolism; D008297:Male; D008870:Microtubules / Q000502:physiology; D020033:Protein Isoforms / Q000378:metabolism; D021381:Protein Transport; D014404:Tubulin / Q000378:metabolism*
D020033:Protein Isoforms; D014404:Tubulin