G-PROTEIN BETA-SUBUNIT NULL MUTANTS ARE IMPAIRED IN PHAGOCYTOSIS AND CHEMOTAXIS DUE TO INAPPROPRIATE REGULATION OF THE ACTIN CYTOSKELETON

Chemotaxis and phagocytosis are basically similar in cells of the immu ne system and in Dictyostelium amebae. Deletion of the unique G protei n beta subunit in D. discoideum impaired phagocytosis but bad little e ffect on fluid-phase endocytosis, cytokinesis, or random motility. Con stitutive expression of wild-type beta subunit restored phagocytosis a nd normal development. Chemoattractants released by cells or bacteria trigger typical transient actin polymerization responses in wild-type cells. In beta subunit-null cells, and in a series of beta subunit poi nt mutants, these responses were impaired to a degree that correlated with the defect in phagocytosis. Image analysis of green fluorescent p rotein-actin transfected cells showed that beta subunit-null cells wer e defective in reshaping the actin network into a phagocytic cup, and eventually a phagosome, in response to particle attachment. Our result s indicate that signaling through heterotrimeric G proteins is require d for regulating the actin cytoskeleton during phagocytic uptake, as p reviously shown for chemotaxis. Inhibitors of phospholipase C and intr acellular Ca2+ mobilization inhibited phagocytosis, suggesting the pos sible involvement of these effecters in the process.