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MBC in Press, published online ahead of print September 15, 2004
Mol. Biol. Cell 10.1091/mbc.E04-03-0254

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Submitted on March 25, 2004
Revised on August 31, 2004
Accepted on September 3, 2004

Septin Ring Assembly Requires Concerted Action of Polarisome Components, a PAK Kinase Cla4p, and the Actin Cytoskeleton in Saccharomyces cerevisiae

Jun Kadota,* Takaharu Yamamoto,* Shiro Yoshiuchi,* Erfei Bi,{dagger} and Kazuma Tanaka*{ddagger}

*Division of Molecular Interaction, Institute for Genetic Medicine, Hokkaido University Graduate School of Medicine, Sapporo 060-0815, Japan; {dagger}Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6058

Monitoring Editor: Tim Stearns

Septins are filament-forming proteins that function in cytokinesis in a wide variety of organisms. In budding yeast, the small GTPase Cdc42p triggers the recruitment of septins to the incipient budding site and the assembly of septins into a ring. We herein report that Bni1p and Cla4p, effectors of Cdc42p, are required for the assembly of the septin ring during the initiation of budding but not for its maintenance after the ring converts to a septin collar. In bni1{Delta} cla4-75-td mutant, septins were recruited to the incipient budding site. However, the septin ring was not assembled, and septins remained at the polarized growing sites. Bni1p, a formin family protein, is a member of the polarisome complex with Spa2p, Bud6p, and Pea2p. All spa2{Delta} cla4-75-td, bud6{Delta} cla4-75-td, and pea2{Delta} cla4-75-td mutants showed defects in septin ring assembly. Bni1p stimulates actin polymerization for the formation of actin cables. Point mutants of BNI1 that are specifically defective in actin cable formation also exhibited septin ring assembly defects in the absence of Cla4p. Consistently, treatment of cla4{Delta} mutant with the actin inhibitor latrunculin-A inhibited septin ring assembly. Our results suggest that polarisome components and Cla4p are required for the initial assembly of the septin ring and that the actin cytoskeleton is involved in this process.

{ddagger}Corresponding author. E-mail: k-tanaka{at}igm.hokudai.ac.jp






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