Fig. 4. Purified phagosomes containing latex beads are free from contamination by endosomes and lysosomes. Cells were incubated with growth medium containing ³⁵S-methionine for 1 hour in order to label proteins. Unlabeled cells (A) were pulsed with latex beads for 20 minutes, washed, mixed with labeled cells and latex bead containing phagosomes were purified as described in Materials and Methods. Labeled cells (B) were treated as described for unlabeled cells in A. Latex beads (C) were added to labeled cell homogenate and then purified from the homogenate as in A. Equal protein loads were separated by SDS-PAGE and ³⁵S-met labeled proteins were visualized by fluorography. H, cell homogenate; P, purified phagosomal membranes. The positions of marker proteins (kDa) are shown.