JCB_201610001 1775..1794

Phagocytic removal of apoptotic cells is important for tissue remodeling, suppression of inflammation, and regulation of immune responses (Savill and Fadok, 2000; Savill et al., 2002). In Caenorhabditis elegans hermaphrodites, 131 somatic cells and ∼50% of germ cells die through apoptosis, and the resulting cell corpses are phagocytosed and cleared by neighboring cells in the soma or by gonadal sheath cells that encase the germ line. During this process, apoptotic cells expose the phosphatidylserine “eat me” signal on the surface and are recognized and engulfed by phagocytes through evolutionarily conserved pathways, leading to cytoskeleton reorganization and formation of membrane-bound vesicles, namely phagosomes (Pinto and Hengartner, 2012; Wang and Yang, 2016). Maturation of cell corpse–enclosing phagosomes, which in many ways parallels endosome progression and maturation of phagosomes containing foreign bodies, involves sequential interactions with early endosomes, late endosomes, and lysosomes to yield phagolysosomes, where apoptotic cells are degraded (Flannagan et al., 2012; Wang and Yang, 2016). As the key regulators of membrane trafficking, Rab GTPases act at multiple steps to mediate various membrane-remodeling events, leading to maturation of phagosomes and formation of phagolysosomes capable of digesting phagosomal contents (Flannagan et al., 2012; Gutierrez, 2013). In worms, four Rabs (RAB-5, UNC-108/Rab2, RAB-14, and RAB-7) function in a stepwise manner to promote phagosome maturation and cell corpse degradation. RAB-5 transiently associates with early phagosomes to promote phosphatidylinositol 3-phosphate (PtdIns3P) generation, probably by activating the phosphoinositide-3 kinase VPS-34, whereas RAB-7 is recruited later to mediate phagolysosome formation, probably through HOPS complex components (Kinchen et al., 2008; Yu et al., 2008; Xiao et al., 2009). Progression of phagosome maturation requires transition from RAB-5–positive early phagosomes to RAB-7–positive late phagosomes. The GTPase-activating protein TBC-2 inactivates RAB-5 to release it from phagosomal membranes, thereby promoting progression of phagosome maturation through the RAB-5–positive stage (Li et al., 2009). In addition, SAND-1/Monl acts with CCZ-1/Ccz1 to Apoptotic cells generated by programmed cell death are engulfed by phagocytes and enclosed within plasma membrane– derived phagosomes. Maturation of phagosomes involves a series of membrane-remodeling events that are governed by the sequential actions of Rab GTPases and lead to formation of phagolysosomes, where cell corpses are degraded. Here we identified gop-1 as a novel regulator of apoptotic cell clearance in Caenorhabditis elegans. Loss of gop-1 affects phagosome maturation through the RAB-5–positive stage, causing defects in phagosome acidification and phagolysosome formation, phenotypes identical to and unaffected by loss of unc-108, the C. elegans Rab2. GOP-1 transiently associates with cell corpse–containing phagosomes, and loss of its function abrogates phagosomal association of UNC108. GOP-1 interacts with GDP-bound and nucleotide-free UNC-108/Rab2, disrupts GDI-UNC-108 complexes, and promotes activation and membrane recruitment of UNC-108/Rab2 in vitro. Loss of gop-1 also abolishes association of UNC-108 with endosomes, causing defects in endosome and dense core vesicle maturation. Thus, GOP-1 is an activator of UNC-108/Rab2 in multiple processes. GOP-1 promotes apoptotic cell degradation by activating the small GTPase Rab2 in C. elegans