Fission yeast Pob1p, which is homologous to budding yeast Boi proteins andexhibits subcellular localization close to actin patches, is essential forcell elongation and separation

The fission yeast pob1 gene encodes a protein of 871 amino acids carrying a n SH3 domain, a SAM domain, and a PH domain. Gene disruption and constructi on of a temperature-sensitive pob1 mutant indicated that pob1 is essential for cell growth. Loss of its function leads to quick cessation of cellular elongation. Pob1p is homologous to two functionally redundant Saccharomyces cerevisiae proteins, Boi1p and Boi2p, which are necessary for cell growth and relevant to bud formation. Overexpression of pob1 inhibits cell growth, causing the host cells to become round and swollen. In growing cells, Pob1 p locates at cell tips during interphase and translocates near the division plane at cytokinesis. Thus, this protein exhibits intracellular dynamics s imilar to F-actin patches. However, Pob1p constitutes a layer, rather than patches, at growing cell tips. It generates two split discs flanking the se ptum at cytokinesis. The pob1-defective cells no longer elongate but swell gradually at the middle, eventually assuming a lemon-like morphology. Analy sis using the pob1-ts allele revealed that Pob1p is also essential for cell separation. We speculate that Pob1p is located on growing plasma membrane, possibly through the function of actin patches, and may recruit proteins r equired for the synthesis of cell wall.