The γ-tubulin complex protein Alp4 provides a link between the metaphase checkpoint and cytokinesis in fission yeast

Background: The progression of cytokinesis requires cyclin B destruction by the anaphase promoting complex (APC/C) and, in fission yeast, activation of the septation initiation network (SIN) is also essential. The gamma-tubulin complex (gamma-TuC) localizes to the centrosome throughout the cell cycle and is directly involved in the organization of the mitotic spindle. Results: We have previously shown that the mutant defective in alp4(+) (Spc97/GCP2) displays bipolar spindle defects due to a failure in the recruitment of the gamma-TuC on to the spindle pole body (SPB, the centrosome equivalent). Here we show that in these mutants the Mad2 checkpoint is activated, yet septation proceeds due to the untimely activation of the SIN. The Sid1 kinase, the downstream effector of the SIN, is recruited prematurely to both, instead of only one, of the SPBs, which triggers septation despite the presence of monopolar spindles. Remarkably, cyclin B levels, which would normally have declined, remain high at the SPB in septated mutant cells. Conclusions: We propose a novel role of the gamma-TuC in inhibiting activation of the SIN until cyclin B is destroyed. Given the ubiquitous existence of the gamma-TuC, this mechanism may be conserved throughout evolution and function to couple cytokinesis to mitotic exit.