Analysis of the MAP kinase, DdERK2, in chemotactic signal transduction of Dictyostelium

Abstract

Dictyostelium mutants lacking the MAP kinase DdERK2 show reduced chemotactic responses to folate and cAMP. Analysis of cAMP chemotaxis shows that Dderk2{dollar}\sp-{dollar} cells are defective in chemotaxis to high concentrations of cAMP. This defect is due to an inability to repolarize in the continued presence of high concentrations of cAMP. Under these conditions, the speed of movement of mutant cells remains low. Instead of generating a leading pseudopod, mutant cells generate transient crown-like structures over multiple regions of the cell surface. These structures differ from pseudopods in that they contain myosin II as well as F actin and coronin. These studies identify a role for MAP kinases in coordinating the formation of cell projections generated in response to chemoattractants. A polyclonal antibody against a MAP kinase (DdERK2) in Dictyostelium has been made and used to study DdERK2 activation and localization. The activation of DdERK2 by chemoattractants cAMP and folate is fast and transient. Its activity peaks between 15 and 60 seconds after cAMP stimulation and declines to basal levels after 5 minutes. In parallel with the DdERK2 activation is the appearance of a higher mobility band on Western blots. An antibody specific for activated MAP kinase shows that only the shifted band is tyrosine phosphorylated, suggesting that it is the active form. Both unstimulated and stimulated DdERK2 are soluble. In vitro phosphorylation with cell lysate supernatants or immunoprecipitates demonstrates the presence of several potential substrates, as identified by SDS-PAGE with mobility corresponding to molecular weights of 150 kDa, 25 kDa and 19 kDa. Furthermore, immunoprecipitation studies suggest that these substrates are in a complex with DdERK2. These data suggest that DdERK2 works via soluble proteins to mediate signaling responses in Dictyostelium.