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.