Abstract
Retromer is an endosomal multi‐protein complex that organizes the endocytic recycling of a vast range of integral membrane proteins. Here, we establish an additional retromer function in controlling the activity and localization of the late endosomal small GTPase RAB7. Surprisingly, we found that RAB7 not only decorates late endosomes or lysosomes, but is also present on the endoplasmic reticulum, trans‐Golgi network, and mitochondrial membranes, a localization that is maintained by retromer and the retromer‐associated RAB7‐specific GAP TBC1D5. In the absence of either TBC1D5 or retromer, RAB7 activity state and localization are no longer controlled and hyperactivated RAB7 expands over the entire lysosomal domain. This lysosomal accumulation of hyperactivated RAB7 results in a striking loss of RAB7 mobility and overall depletion of the inactive RAB7 pool on endomembranes. Functionally, we establish that this control of RAB7 activity is not required for the recycling of retromer‐dependent cargoes, but instead enables the correct sorting of the autophagy related transmembrane protein ATG9a and autophagosome formation around damaged mitochondria during Parkin‐mediated mitophagy.
Highlights
The ancient and evolutionarily conserved retromer complex is an endosomal multi-protein assembly that orchestrates the endocytic recycling of a vast range of transmembrane proteins, either from endosomes to the trans-Golgi network (TGN) or from endosomes to the plasma membrane (Burd & Cullen, 2014)
Expressed GFP-RAB7a localized to tubular structures which partially overlapped with mitochondrial TOM20, confirming the antibody data (Fig EV1E)
GFP-RAB7a that was expressed with a lentivirus in RAB7a KO cells localized to vesicles and to tubular networks that partially co-localized with MitoTracker Red in living HeLa cells (Fig EV1G and Movie EV1), which rules out that fixation artifacts caused the mitochondria and ER localization of RAB7
Summary
The ancient and evolutionarily conserved retromer complex is an endosomal multi-protein assembly that orchestrates the endocytic recycling of a vast range of transmembrane proteins, either from endosomes to the trans-Golgi network (TGN) or from endosomes to the plasma membrane (Burd & Cullen, 2014). The heterotrimeric core of the retromer complex, composed of the proteins VPS26, VPS29, and VPS35 (Seaman et al, 1998), localizes to the endosomal membrane via simultaneous binding of the VPS35 subunit to active RAB7-GTP and to the sorting nexin SNX3 (Rojas et al, 2008; Seaman et al, 2009; Balderhaar et al, 2010; Liu et al, 2012; Harrison et al, 2014). Retromer is known to be a RAB7 effector that depends on this small GTPase for its localization to the endosomal membrane (Seaman et al, 2009; Harrison et al, 2014), the precise relationship or any potential cross-regulation between retromer and RAB7 has not been investigated in depth.
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