Aggregation of Dictyostelium discoideum is dependent on myristoylation and membrane localization of the G protein α-subunit, Gα2

Abstract

The heterotrimeric G protein, G2, from the eukaryotic organism Dictyostelium discoideum participates in signal transduction pathways which are essential to Dictyostelium's developmental life cycle. G2 is activated by cell surface cAMP receptors and in turn is required for the activation of a host of effectors, including adenylyl cyclase, guanylyl cyclase, and phospholipase C. Myristoylation of G protein α-subunits is known to affect α-subunit association with the βγ subunits and membrane localization. The putative site for N-terminal myristoylation of Gα2 was mutated from Gly to Ala (G2A) and expressed in the gα2-null cell line, MYC2. Transformants expressing Gα2-G2A exhibit physiological and biochemical changes from wild- type cells. Gα2-G2A expressing cells fail to rescue the aggregation-minus phenotype of MYC2 cells on developmental agar plates. Gα2-G2A expressing cells are also not chemotactic to cAMP in a standard drop assay. Gα2-WT is found in both the pellet and supernatant fractions following lysis of the cells. Gα2-G2A however is found almost exclusively in the lysate supernatant. Gα2 is radiolabeled upon incubation of cells in [3H]myristate, while Gα2-G2A is not labeled. Examination of activation of the effectors adenylyl cyclase and guanylyl cyclase reveals that Gα2-G2A expressing cells partially activate adenylyl cyclase but show no cAMP-stimulation of guanylyl cyclase. The physiological deviations from wild-type can be explained by the variations in effector activation, possibly due to improper localization of the non-myristoylated Gα2-G2A to the cytosol.