Molecular characterization of phagosomes.

The transformation of newly formed phagosomes into mature phagolysosomes is a process that involves a complex series of interactions between phagosomes and other vacuolar organelles. The machinery required by phagosomes to mediate these interactions is poorly understood. In this study, we allowed human and various rodent cells to take up latex beads whose density facilitates a simple purification of phagosomes using discontinuous sucrose gradients. With this system, we initiated a systematic study of phagosome proteins using two-dimensional gel electrophoresis and the currently available two-dimensional gel protein data bases. By this approach, we were able to recognize a group of polypeptides associated with mouse J774 phagosomes-phagolysosomes including annexin II, annexin VI, the beta-1 and beta-2 subunits of trimeric G proteins, and a group of actin-binding proteins. While the amount of annexin II associated to phagosomes was similar at all times of latex internalization, the levels of annexin VI were higher on late phagosomes. Phospholipid analysis of J774 phagosomes isolated at early and late time points during phagolysosome formation also revealed significant differences in their lipid composition. In the human phagosomes, we resolved over 200 polypeptides on the two-dimensional gels. These included the proteins described in the mouse, as well as 32 polypeptides that were found to be highly enriched in phagosomes, 15 of which are not present in the current data bases. The results demonstrate that the use of latex bead phagosomes is a powerful system to identify key molecules involved in phagolysosome biogenesis.