Vol. 12, Issue 7, 2185-2194, July 2001

Tubulin Sorting during Dimerization In Vivo

Henry D. Hoyle,^* F. Rudolf Turner, Linda Brunick, and Elizabeth C. Raff

Department of Biology and Indiana Molecular Biology Institute, Indiana University, Bloomington, Indiana 47405

We demonstrate sorting of -tubulins during dimerization in the Drosophila male germ line. Different -tubulin isoforms exhibit distinct affinities for -tubulin during dimerization. Our data suggest that differences in dimerization properties are important in determining isoform-specific microtubule functions. The differential use of -tubulin during dimerization reveals structural parameters of the tubulin heterodimer not discernible in the resolved three-dimensional structure. We show that the variable -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 - heterodimer. If the availability of -tubulin is limiting, - dimers preferentially incorporate intact -tubulins rather than a -tubulin missing the carboxyl terminus (2C). When -tubulin is not limiting, 2C forms stable - heterodimers. Once dimers are formed, no further sorting occurs during microtubule assembly: -2C dimers are incorporated into axonemes in proportion to their contribution to the total dimer pool. Co-incorporation of 2C and wild-type 2-tubulin results in nonmotile axonemes because of a disruption of the periodicity of nontubulin axonemal elements. Our data show that the -tubulin carboxyl terminus has two distinct roles: 1) forming the - heterodimer, important for all microtubules and 2) providing contacts for nontubulin components required for specific microtubule structures, such as axonemes.