Sid4p is required to localize components of the septation initiation pathway to the spindle pole body in fission yeast

A mutation in the Schizosaccharomyces pombe sid4(+) (septation initiation d efective) gene was isolated in a screen for mutants defective in cytokinesi s. We have cloned sid4+ and have found that sid4+ encodes a previously unkn own 76.4-kDa protein that localizes to the spindle pole body (SPB) througho ut the cell cycle. Sid4p is required for SPB localization of key regulators of septation initiation, including the GTPase Spg1p. the protein kinase Cd c7p. and the GTPase-activating protein Byr4p. An N-terminally truncated Sid 4p mutant does not localize to SPBs and when overproduced acts as a dominan t-negative mutant by titrating endogenous Sid4p and Spg1p from the SPB. Con versely, the Sid4p N-terminal 153 amino acids are sufficient for SPB locali zation. Biochemical studies demonstrate that Sid4p interacts with itself, a nd yeast two-hybrid analysis shows that its serf-interaction domain lies wi thin the C-terminal half of the protein. Our data indicate that Sid4p SPB l ocalization is a prerequisite for the execution of the Spg1p signaling casc ade.