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 (beta2 DeltaC). When alpha -tubulin is not limiting, beta2 DeltaC forms stable alpha-beta heterodimers. Once dimers are formed, no further sorting occurs during microtubule assembly: alpha-beta2 DeltaC dimers are incorporated into axonemes in proportion to their contribution to the total dimer pool. Co-incorporation of beta2 DeltaC and wild-type beta2-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.