Assembly and regulation of signaling proteins at fission yeast microtubule organizing centers

Abstract

The spindle pole body, the yeast analog of the centrosome, serves not only to nucleate and organize microtubules but also as a signaling center to coordinate events in mitosis and cytokinesis. It does so by localizing proteins responsible for chromosome segregation, spindle formation and cytokinesis. The first part of my study focuses on the signaling pathway, SIN (septation initiation network), located at the spindle pole body and is responsible for initiating actomyosin ring constriction, septation and cell division. In an effort to identify novel components or tethers of the in the SIN to the SPB, we performed a TAP (tandem purification analysis) analysis on Cdc11p, an essential SIN scaffolding protein and identified a previously uncharacterized protein, Ppc89. Ppc89 localizes constitutively to the SPB and interacts directly with Sid4. ppc89? cells are inviable and exhibit defects in SPB integrity, and hence in spindle formation, chromosome segregation, and SIN localization. Ppc89 overproduction is lethal, resulting primarily in a G2 arrest accompanied by massive enlargement of the SPB and increased SPB MT nucleation. These results suggest a fundamental role for Ppc89 in organization of the S. pombe SPB. The second part of my studies focused on characterizing the role of phosphorylation on Mto2, a protein that activates the ?-TuC and localizes it to iMTOCs and eMTOCs during interphase. However, it is not known how Mto2 performs this function. Based on previous studies, we hypothesized that Mto2 could possibly be phospho-regulated in a cell-cycle dependent manner. To test this hypothesis, I examined Mto2 throughout the cell cycle and found that Mto2 is hyperphosphorylated during mitosis by Cdk1. Mutation of these sites to nonphosphorylatable alanine residues eliminates the mitotic phosphorylation but does not alter function. We therefore hypothesize that the mitotic phosphorylation inhibits Mto2 from activating the ?-TuC.