PAKa, a putative PAK family member, is required for cytokinesis and the regulation of the cytoskeleton in Dictyostelium discoideum cells during chemotaxis

We have identified a Dictyostelium? discoideum gene encoding a serine/threo nine kinase, PAKa, a putative member of the Ste20/PAK family of p21-activat ed kinases, with a kinase domain and a long NH2-terminal regulatory domain containing an acidic segment, a polyproline domain, and a CRIB domain. PAKa colocalizes with myosin II to the cleavage furrow of dividing cells and th e posterior of polarized, chemo-taxing cells via its NH2-terminal domain. p aka null cells are defective in completing cytokinesis in suspension. PAKa is also required for maintaining the direction of cell movement, suppressin g lateral pseudopod extension, and proper retraction of the posterior of ch emotaxing cells, paka null cells are defective in myosin II assembly, as th e myosin II cap in the posterior of chemotaxing cells and myosin II assembl y into cytoskeleton upon cAMP stimulation are absent in these cells, while constitutively active PAKa leads to an upregulation of myosin II assembly. PAKa kinase activity against histone 2B is transiently stimulated and PAKa incorporates into the cytoskeleton with kinetics similar to those of myosin II assembly in response to chemoattractant signaling. However, PAKa does n ot phosphorylate myosin II. We suggest that PAKa is a major regulator of my osin II assembly, but does so by negatively regulating myosin II heavy chai n kinase.