C. elegansRab GTPase activating protein TBC-2 promotes cell corpse degradation by regulating the small GTPase RAB-5

Abstract

During apoptosis, dying cells are quickly internalized by neighboring cells or phagocytes, and are enclosed in phagosomes that undergo a maturation process to generate the phagoslysosome, in which cell corpses are eventually degraded. It is not well understood how apoptotic cell degradation is regulated. Here we report the identification and characterization of the C. elegans tbc-2 gene, which is required for the efficient degradation of cell corpses. tbc-2 encodes a Rab GTPase activating protein (GAP) and its loss of function affects several events of phagosome maturation, including RAB-5 release, phosphatidylinositol 3-phosphate dynamics, phagosomal acidification, RAB-7 recruitment and lysosome incorporation, which leads to many persistent cell corpses at various developmental stages. Intriguingly, the persistent cell corpse phenotype of tbc-2 mutants can be suppressed by reducing gene expression of rab-5, and overexpression of a GTP-locked RAB-5 caused similar defects in phagosome maturation and cell corpse degradation. We propose that TBC-2 functions as a GAP to cycle RAB-5 from an active GTP-bound to an inactive GDP-bound state, which is required for maintaining RAB-5 dynamics on phagosomes and serves as a switch for the progression of phagosome maturation.

Rab GTPases and tethering in the yeast endocytic pathway

“…In C. elegans Mon1 can shut down the previous GEF and TBC-2 has been identified as a GAP for RAB-5, that needs RAB-7 to localize on the late endosome. 22,23 Mammalian RabGAP-5 specifically blocks the EGF uptake in early endocytosis by deactivating Rab5. 24 In yeast, no specific GAP for Vps21 has been identified so far.…”

Differential Effects of TBC1D15 and Mammalian Vps39 on Rab7 Activation State, Lysosomal Morphology, and Growth Factor Dependence

“…Both a large, hexameric Vps39-containing complex and Ypt7 are required for docking (29, 31). The protein complex containing Vps39, Vps41, and the homotypic fusion and vacuole protein sorting (HOPS) complex (Vps11,16,18,and 33) may function as a Ypt7 effector in yeast, linking Rab7-GTP to the trans-SNARE pairing that completes the docking stage. After docking, membrane fusion can proceed in the absence of Ypt7 (21).…”

“…Multiple Rab GAPs contain a TBC (Tre2/Bub2/Cdc16) domain (32)(33)(34)(35)(36)(37)(38). TBC1D15 has been identified as a putative Rab7 GAP based on its homology to Gyp7.…”

Sequential action of Caenorhabditis elegans Rab GTPases regulates phagolysosome formation during apoptotic cell degradation

“…GAPEX5, a VPS9 domain-containing RAB5 GEF, is recruited to phagosomes by a microtubule-dependent mechanism and regulates RAB5 activation on apoptotic cell-containing phagosomes in Swiss 3T3 cells (5). In C. elegans, the GTPase-activating protein TBC-2 inactivates RAB-5 to release it from phagosomal membranes, thereby promoting the progression of phagosome maturation through the RAB-5-positive stage (7).…”