Clathrin and the multi-subunit adaptor protein complex AP2 are central players in clathrin-mediated endocytosis by which the cell selectively internalizes surface materials. Here, we report the essential role of clathrin and AP2 in phagocytosis of apoptotic cells. In Caenorhabditis elegans, depletion of the clathrin heavy chain CHC-1 and individual components of AP2 led to a significant accumulation of germ cell corpses, which resulted from defects in both cell corpse engulfment and phagosome maturation required for corpse removal. CHC-1 and AP2 components associate with phagosomes in an inter-dependent manner. Importantly, we found that the phagocytic receptor CED-1 interacts with the α subunit of AP2, while the CED-6/Gulp adaptor forms a complex with both CHC-1 and the AP2 complex, which likely mediates the rearrangement of the actin cytoskeleton required for cell corpse engulfment triggered by the CED-1 signaling pathway. In addition, CHC-1 and AP2 promote the phagosomal association of LST-4/Snx9/18/33 and DYN-1/dynamin by forming a complex with them, thereby facilitating the maturation of phagosomes necessary for corpse degradation. These findings reveal a non-classical role of clathrin and AP2 and establish them as indispensable regulators in phagocytic receptor-mediated apoptotic cell clearance.
Phagocytosis of apoptotic cells is an indispensable part of the cell death program. During phagocytosis, the evolutionarily conserved CED-1 family phagocytic receptors recognize cell corpses and transduce engulfment signals to induce the formation and maturation of phagosomes. However, it remains largely unknown how the CED-1 signaling pathway induces the cytoskeletal reorganization required for corpse internalization and initiates phagosome maturation. Interestingly, cell corpse phagocytosis appears to resemble clathrin-mediated endocytosis (CME) of cell surface molecules in that both events cause receptor-dependent internalization of extracellular cargoes differing only in size. In CME, the recognition of plasma membrane receptors by adaptor proteins such as the AP2 complex triggers the formation of clathrin-coated vesicles (CCVs) with diameters ranging from 10–200 nm. Nevertheless, it is not known whether clathrin and AP2 also play a role in phagocytosis of apoptotic cells that are much larger than CCVs. Here we provide genetic, cell biological, and biochemical experimental findings to demonstrate that clathrin and AP2 act downstream of CED-1 to regulate the actin cytoskeleton rearrangement required for cell corpse internalization and cell corpse degradation. Clathrin and AP2 form two types of complex with factors required for engulfment and phagosome maturation. These findings establish clathrin and AP2 as essential players in phagocytic receptor-mediated apoptotic cell clearance.