The fission yeast γ-tubulin complex is required in G1 phase and is a component of the spindle assembly checkpoint

Microtubule polymerization is initiated from the microtubule organizing centre (MTOC), which contains the gamma -tubulin complex. We have identified fission yeast Alp4 and Alp6, which are homologues of the gamma -tubulin-interacting proteins Sc.Spc97/Hs.Gcp2 and Sc.Spc98/Hs.Gcp3, respectively. The size of the fission yeast gamma -tubulin complex is large (>2000 kDa), comparable to that in metazoans. Both Alp4 and Alp6 localize to the spindle pole body (SPB) and also to the equatorial MTOC. Temperature-sensitive (ts) alp4 and alp6 mutants show two types of microtubular defects. First, monopolar mitotic spindles form. Secondly, abnormally long cytoplasmic microtubules appear that do not stop at the cell tips and are still associated with the SPB. Alp4 function is required in G(1) phase and ts mutants become lethal before S-phase. alp4 and alp6 mutants are hypersensitive to the microtubule-destabilizing drug thiabendazole (TBZ) and show a lethal 'cut' phenotype in its presence. Furthermore, alp4mad2 double mutants show an exaggerated multiple septation phenotype in TBZ. These results indicate that Alp4 and Alp6 may play a crucial role in the spindle pole-mediated checkpoint pathway.