Analysis of a putative map kinase docking motif in Dictyostelium discoideum Ga2 protein

Abstract

Some G protein alpha subunits contain a mitogen-activated protein kinase (MAPK) docking site (D-motif) near the amino terminus that can impact cellular responses to external signals. The Dictyostelium Ga2 subunit is required for chemotaxis to cAMP during the onset of multicellular development and the subunit contains a putative D-motif in a region analogous to that in other Ga subunits. The Ga2 subunit D-motif was altered (Ga2^D-) to examine its potential role in chemotaxis and multicellular development. In ga2- cells the expression of the Ga2^D- or wild-type Ga2 subunit from high copy number vectors rescued cell aggregation but blocked the transition of mounds into slugs. This phenotype was also observed in parental strains with a wild-type Ga2 locus indicating that the heterologous Ga2 subunit expression interferes with multicellular developmental progress. Expression of the Ga2^D- subunit from a low copy number vector in ga2- cells did not rescue aggregation whereas the wild-type Ga2 subunit rescued aggregation efficiently and allowed wild-type morphological development. The Ga2^D- and Ga2 subunit were both capable of restoring comparable levels of cAMP chemotaxis and the ability to co-aggregate with wild-type cells implying that Ga2^D- expressing cells are defective in intercellular signaling. The ability of cAMP to stimulate the translocation of the GtaC transcription factor was impaired in Ga2^D- expressing cells compared to Ga2 expressing cells suggesting the putative D-motif is important for developmental gene regulation. These results suggest that the D-motif plays a role in aggregation and some cellular responses to cAMP but not cAMP chemotaxis.