SIGNAL-TRANSDUCTION AND MOTILITY OF DICTYOSTELIUM

This review is concerned with the roles of cyclic GMP and Ca2+ ions in signal transduction for chemotaxis of Dictyostelium. These molecules are involved in signalling between the cell surface cyclic AMP recepto rs and cytoskeletal myosin II involved in chemotactic cell movement. E vidence is presented for uptake and/or eflux of Ca2+ being regulated b y cyclic GMP. The link between Ca2-, cyclic GMP and chemotactic cell m ovement has been explored using ''streamer F'' mutants whose primary d efect is in the structural gene for the cyclic GMP-specific phosphodie sterase. This mutation causes the mutants to produce an abnormally pro longed peak of cyclic GMP accumulation in response to stimulation with the chemoattractant cyclic AMP. The production and relay of cyclic AM P signals is normal in these mutants, but certain events associated wi th movement are (like the cyclic GMP response) abnormally prolonged in the mutants. These events include Ca2+ uptake, myosin II association with the cytoskeleton and regulation of both myosin heavy and light ch ain phosphorylation. These changes can be correlated with changes in t he shape of the amoebae after chemotactic stimulation. Other mutants i n which the accumulation of cyclic GMP in response to cyclic AMP stimu lation was absent produced no myosin II responses. A model is describe d in which cyclic GMP (directly or indirectly via Ca2+) regulates accu mulation of myosin II on the cytoskeleton by regulating phosphorylatio n of the myosin heavy and light chain kinases.