DICTYOSTELIUM-DISCOIDEUM MUTANTS WITH CONDITIONAL DEFECTS IN PHAGOCYTOSIS

We have isolated and characterized Dictyostelium discoideum mutants wi th conditional defects in phagocytosis. Under suspension conditions, t he mutants exhibited dramatic reductions in the uptake of bacteria and polystyrene latex beads. The initial binding of these ligands was una ffected, however, indicating that the defect was not in a plasma membr ane receptor. Because of the phagocytosis defect, the mutants were una ble to grow when cultured in suspensions of heat-killed bacteria. The mutants exhibited normal capacities for fluid phase endocytosis and gr ew as rapidly as parental (AX4) cells in axenic medium. Both the defec ts in phagocytosis and growth on bacteria were corrected when the muta nt Dictyostelium cells were cultured on solid substrates. Reversion an d genetic complementation analysis suggested that the mutant phenotype s were caused by single gene defects. While the precise site of action of the mutations was not established, the mutations are likely to aff ect an early signaling event because the binding of bacteria to mutant cells in suspension was unable to trigger the localized polymerizatio n of actin filaments required for ingestion; other aspects of actin fu nction appeared normal. This class of conditional phagocytosis mutant should prove to be useful for the expression cloning of the affected g ene(s).