Novel potential mitotic motor protein encoded by the fission yeast cut7+ gene

THE structure equivalent to higher eukaryotic centrosomes in fission yeast, the nuclear membrane-bound spindle pole body, is inactive during interphase. On transition from G2 to M phase of the cell cycle, the spindle pole body duplicates; the daughter pole bodies seed microtubules which interdigitate to form a short spindle that elongates to span the nucleus at metaphase^1-5. We have identified two loci which, when mutated, block spindle formation. The predicted product of one of these genes, cut7⁺ (ref. 6), contains an amino-terminal domain similar to the kinesin heavy chain head domain^7,8, indicating that the cut7⁺ product could be a spindle motor. The cut7⁺ gene resembles the Aspergillus nidulans putative spindle motor gene bimC (ref. 9), both in terms of its organization with a homologous amino-terminal head and no obvious heptad repeats and in the morphology of the mutant phenotype. But we find no similarity between the carboxy termini of these genes, suggesting that either the cut7⁺ gene represents a new class of kinesin genes and that fission yeast may in addition contain a bimC homologue, or that the carboxy termini of these mitotic kinesins are not evolutionary conserved and that the cut7⁺ gene belongs to a subgroup of bimC-related kinesins.