Figure 2.

Life Cycle of the Phagosome. Phagosomes are formed on demand when professional phagocytes encounter target particles such as, opsonized bacteria or apoptotic cells. Following internalization of a phagocytic target, the nascent phagosome is transformed into a microbiocidal vacuole through successive fusion and fission events within the endocytic pathway. During this maturation process, the lumen of the phagosome is acidified by the action of v-ATPase, and also acquires hydrolases and other antimicrobial proteins. Key steps in the maturation process include the conversion from a Rab5-positive early phagosome to a Rab7-positive late phagosome; the acquisition of LAMP and v-ATPase; the conversion of PI3P to PI3,5P₂; and the appearance of PI4P. Once the particle within the phagolysosome has been degraded, PI4P- and Arl8b-dependent membrane tubulation and budding events breakdown the organelle, reforming lysosomes consumed in the maturation process. EEA1, early endosomal antigen 1; LAMP, lysosome-associated membrane protein; PI3P, phosphatidylinositol 3-phosphate; PI3,5P₂, phosphatidylinositol 3,5- bisphosphate; PI4P, phosphatidylinositol 4-phosphate; Arl8b, ADP-ribosylation factor-like protein 8B.