Conditional Mutations in γ-Tubulin Reveal Its Involvement in Chromosome Segregation and Cytokinesis

γ-Tubulin is a conserved essential protein required for assembly and function of the mitotic spindle in humans and yeast. For example, human γ-tubulin can replace the γ-tubulin gene in Schizosaccharomyces pombe. To understand the structural/functional domains of γ-tubulin, we performed a systematic alanine-scanning mutagenesis of human γ-tubulin (TUBG1) and studied phenotypes of each mutant allele in S. pombe. Our screen, both in the presence and absence of the endogenous S. pombe γ-tubulin, resulted in 11 lethal mutations and 12 cold-sensitive mutations. Based on structural mapping onto a homology model of human γ-tubulin generated by free energy minimization, all deleterious mutations are found in residues predicted to be located on the surface, some in positions to interact with α- and/or β-tubulins in the microtubule lattice. As expected, one class of tubg1 mutations has either an abnormal assembly or loss of the mitotic spindle. Surprisingly, a subset of mutants with abnormal spindles does not arrest in M phase but proceeds through anaphase followed by abnormal cytokinesis. These studies reveal that in addition to its previously appreciated role in spindle microtubule nucleation, γ-tubulin is involved in the coordination of postmetaphase events, anaphase, and cytokinesis.