TI - Myosin I heavy chain kinase : cloning of the full-length gene and acidic lipid -dependent activation by Rac and Cdc42 . AB - Acanthamoeba myosin I heavy chain kinase ( MIHCK ) phosphorylates the heavy chains of amoeba myosins I , increasing their actin-activated ATPase activities . The activity of MIHCK is increased by binding to acidic phospholipids or membranes and by autophosphorylation at multiple sites . Phosphorylation at a single site is necessary and sufficient for full activation of the expressed catalytic domain . The rate of autophosphorylation of native MIHCK is controlled by a region N-terminal to the catalytic domain . By its substrate specificity and the sequence of its C-terminal catalytic domain , MIHCK was identified as a p21-activated kinase ( PAK ) . We have now cloned the full-length genomic DNA and cDNA of MIHCK and have shown it to contain the conserved p21-binding site common to many members of the PAK family . Like some mammalian PAKs , MIHCK is activated by Rac and Cdc42 , and this activation is GTP -dependent and accompanied by autophosphorylation . In contrast to mammalian PAKs , activation of MIHCK by Rac and Cdc42 requires the presence of acidic lipids . Also unlike mammalian PAK , MIHCK is not activated by sphingosine or other non-negatively charged lipids . The acidic lipid -binding site is near the N terminus followed by the p21-binding region . The N-terminal regulatory domain of MIHCK contains alternating strongly positive and strongly negative regions.and the extremely Pro-rich middle region of MIHCK has a strongly acidic N-terminal segment and a strongly basic C-terminal segment . We propose that autophosphorylation activates MIHCK by neutralizing the basic segment of the Pro-rich region , thus unfolding the regulatory domain and abolishing its inhibition of the catalytic domain .