Cytokinesis and septation in the fission yeast Schizosaccharomyces pombe are studied as a model for mammalian cell division. In fission yeast, septation is positively regulated by
Spg1, a Ras family
GTPase that localizes to spindle-pole bodies (SPBs) throughout the cell cycle. As cells enter mitosis,
Spg1 accumulates in an active,
GTP-bound form and binds the
Cdc7 protein kinase to cause Cdc7 translocation to SPBs. Cdc7 disappears from one SPB in mid-anaphase and from the second SPB in late mitosis. Byr4 plus Cdc16 negatively regulate septation by forming a two-component
GTPase-activating protein for
Spg1. These results led us to hypothesize that Byr4 localization to SPBs regulated the
nucleotide state of
Spg1, due to its ability to form Spg1GAP activity with Cdc16 and thus the binding of Cdc7 to
Spg1 at SPBs. To test this hypothesis, Byr4 localization was determined using indirect immunofluorescence. This analysis revealed that Byr4 was localized to SPBs that did not contain Cdc7. In byr4(-) mutants, Cdc7 localized to interphase SPBs and only symmetrically localized to mitotic SPBs. In contrast, Byr4 overexpression prevented
Spg1 and Cdc7 localization to SPBs. These results suggest that Byr4 localization to SPBs maintains
Spg1 in an inactive form, presumably by stimulating
Spg1 GTPase activity with Cdc16, and that loss of Byr4 from mitotic SPBs increases the active fraction of
Spg1 and thereby increases Spg1-Cdc7 binding. Byr4 localization to SPBs was decreased in
spg1, cdc16, sid4, and cdc11 mutants as well as in several mutants that affect medial
F-actin structures, suggesting that multiple pathways regulate Byr4 localization to SPBs.