The role of cyclic GMP in regulating myosin during chemotaxis of Dictyostelium: evidence from a mutant lacking the normal cyclic GMP response to cyclic AMP.

Evidence has previously been reported that, during chemotaxis of the cellular slime mould Dictyostelium discoideum, cyclic GMP regulates the association of myosin II with the cytoskeleton and that this regulation is effected by inhibiting myosin II heavy chain phosphorylation (Liu and Newell, J. Cell Sci., 90, 123-129, 1988; 98, 483-490, 1991). Here we provide further evidence in support of this hypothesis using a mutant (KI-10) that is defective in chemotaxis and lacks the normal cyclic AMP-induced cyclic GMP response. We found that the cyclic AMP-induced cytoskeletal actin response was similar to that of the parental strain in this mutant (although showing a slight displacement in the dose-response curve) but the cytoskeletal myosin II heavy chain response was abolished. Moreover, the mutant showed no phosphorylation of myosin II heavy chain in response to cyclic AMP. Compared to the parental strain XP55, the mutant cells contained approximately 40% more protein and their doubling time was 30% longer. These differences could be due to differences in the efficiency of cell division, a process in which the proper regulation of myosin function is essential and in which cyclic GMP may therefore play a role.