Developmental morphology and chemotactic responses are dependent on G alpha subunit specificity in Dictyostelium.

Dictyostelium discoideum expresses multiple G alpha subunits but only a single G beta and G gamma subunit suggesting that the specific response to an external signal depends largely on G alpha subunit function or G protein-independent signaling from the receptor. To test the contribution of G alpha subunit functional specificity, the chimeric G alpha subunits, G alpha2/4 and G alpha5/4, were created and analyzed along with wild-type subunits for the ability to substitute for the G alpha4 subunit in mediating responses from folate receptors. The G alpha2/4 subunit, but not the G alpha2 or G alpha5/4 subunits, partly rescued chemotaxis and cGMP accumulation in folate-stimulated g alpha4(-) cells. Expression of the G alpha5/4 or G alpha5 subunits resulted in an inhibition of g alpha4- and wild-type cell movement and a reduced aggregate size in developing wild-type and g alpha5- cells suggesting these subunits mediate similar responses. Only the G alpha4 subunit was capable of correcting developmental morphology in g alpha4- multicellular aggregates suggesting that the chimeric G alpha2/4 or G alpha5/4 subunits were insufficient to provide the G alpha4 function necessary for proper development. These results indicate that Dictyostelium G alpha subunit specificity is not limited to receptor coupling and that G alpha subunit sequences outside of the carboxyl terminus are important for cell movement and developmental processes.