PHAGOSOMAL PROTEINS OF DICTYOSTELIUM-DISCOIDEUM

In recognizing food particles, Dictyostelium cell-surface molecules in itiate cytoskeletal rearrangements that result in phagosome formation. After feeding D. discoideum cells latex beads, early phagosomes were isolated on sucrose step gradients. Protein analyses of these vesicles showed that they contained glycoproteins and surface-labeled species corresponding to integral plasma membrane proteins. Cytoskeletal prote ins also were associated with phagosomes, including myosin II, actin a nd a 30 kDa-actin bundling protein. As seen by the acridine orange flu orescence of vesicles containing bacteria, phagosomes were acidified r apidly by a vacuolar H+-ATPase that was detected by immunoblotting. Ex cept for the loss of cytoskeletal proteins, few other changes over tim e were noted in the protein profiles of phagosomes, suggesting that ph agosome maturation was incomplete. The indigestibility of the beads po ssibly inhibited further endocytic processing, which has been observed by others. Since nascent phagosomes contained molecules of both the c ytoskeleton and plasma membrane, they will be useful in studies aimed at identifying specific protein associations occurring between membran e proteins and the cytoskeleton during phagocytosis.