Schizosaccharomyces pombe Pak-related protein, Pak1p/Orb2p, phosphorylates myosin regulatory light chain to inhibit cytokinesis

P21 -activated kinases (Paks) have been identified in a variety of eukaryotic cells as key effectors of the Cdc42 family of guanosine triphosphatases. Pak kinases play important roles in regulating the filamentous actin cytoskeleton. In this study, we describe a function for the Schizosaccharomyces pombe Pak-related protein Pak1 p/Orb2p in cytokinesis. Pak1 p localizes to the acto-myosin ring during mitosis and cytokinesis. Loss of Pak1 p function leads to accelerated cytokinesis. Pak1 p mediates phosphorylation of myosin II regulatory light chain Rlc1p at serine residues 35 and 36 in vivo. Interestingly, loss of Pak1 p function or substitution of serine 35 and serine 36 of Rlc1 p with alanines, thereby mimicking a dephosphory-lated state of Rlc1p, leads to defective coordination of mitosis and cytokinesis. This study reveals a new mechanism involving Pak1p kinase that helps ensure the fidelity of cytokinesis. © 2008 Loo and Balasubramanian.