A novel cGMP signalling pathway mediating myosin phosphorylation and chemotaxis in Dictyostelium

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Abstract
Chemotactic stimulation of Dictyostelium cells results in a transient increase in cGMP levels, and transient phosphorylation of myosin II heavy and regulatory light chains. In Dictyostelium , two guanylyl cyclases and four candidate cGMP‐binding proteins (GbpA–GbpD) are implicated in cGMP signalling. GbpA and GbpB are homologous proteins with a Zn2+‐hydrolase domain. A double gbpA/gbpB gene disruption leads to a reduction of cGMP‐phosphodiesterase activity and a 10‐fold increase of basal and stimulated cGMP levels. Chemotaxis in gbpA−B− cells is associated with increased myosin II phosphorylation compared with wild‐type cells; formation of lateral pseudopodia is suppressed resulting in enhanced chemotaxis. GbpC is homologous to GbpD, and contains Ras, MAPKKK and Ras‐GEF domains. Inactivation of the gbp genes indicates that only GbpC harbours high affinity cGMP‐binding activity. Myosin phosphorylation, assembly of myosin in the cytoskeleton as well as chemotaxis are severely impaired in mutants lacking GbpC and GbpD, or mutants lacking both guanylyl cyclases. Thus, a novel cGMP signalling cascade is critical for chemotaxis in Dictyostelium , and plays a major role in myosin II regulation during this process.