NCD AND KINESIN MOTOR DOMAINS INTERACT WITH BOTH ALPHA-TUBULIN AND BETA-TUBULIN

Motor domains of the Drosophila minus-end-directed microtubule (MT) mo tor protein ncd, were found to saturate microtubule binding sites at a stoichiometry of approximately one motor domain per tubulin dimer, To determine the tubulin subunit(s) involved in binding to ncd, mixtures of ncd motor domain and MTs were treated with the zero-length cross-l inker 1-ethyl-3-(3-dimethylaminopropylcarbodiimide) (EDC). EDC treatme nt generated covalently cross-linked products of ncd and alpha-tubulin and of ncd and beta-tubulin, indicating that tile ncd motor domain in teracts with both alpha- and beta-tubulin. When the Drosophila kinesin motor domain protein was substituted for the ncd motor domain, cross- linked products of kinesin and alpha-tubulin and of kinesin and beta-t ubulin were produced. EDC treatment of mixtures of ncd motor domain an d unassembled tubulin dimers or of kinesin motor domain and unassemble d tubulin dimers produced the same motor-tubulin products generated in the presence of MTs. These results indicate that kinesin family motor s of opposite polarity interact with both tubulin monomers and support a model in which some portion of each protein's motor domain overlaps adjacent alpha- and beta-tubulin subunits.