The integral function of the endocytic recycling compartment is regulated by RFFL-mediated ubiquitylation of Rab11 effectors

Endocytic trafficking is regulated by ubiquitylation (also known as ubiquitination) of cargoes and endocytic machineries. The role of ubiquitylation in lysosomal delivery has been well documented, but its role in the recycling pathway is largely unknown. Here, we report that the ubiquitin (Ub) ligase RFFL regulates ubiquitylation of endocytic recycling regulators. An RFFL dominant-negative (DN) mutant induced clustering of endocytic recycling compartments (ERCs) and delayed endocytic cargo recycling without affecting lysosomal traffic. A BioID RFFL interactome analysis revealed that RFFL interacts with the Rab11 effectors EHD1, MICALL1 and class I Rab11-FIPs. The RFFL DN mutant strongly captured these Rab11 effectors and inhibited their ubiquitylation. The prolonged interaction of RFFL with Rab11 effectors was sufficient to induce the clustered ERC phenotype and to delay cargo recycling. RFFL directly ubiquitylates these Rab11 effectors in vitro, but RFFL knockout (KO) only reduced the ubiquitylation of Rab11-FIP1. RFFL KO had a minimal effect on the ubiquitylation of EHD1, MICALL1, and Rab11-FIP2, and failed to delay transferrin recycling. These results suggest that multiple Ub ligases including RFFL regulate the ubiquitylation of Rab11 effectors, determining the integral function of the ERC.

Distinct patterns of phosphatidylserine localization within the Rab11a-containing recycling system

The Rab11 GTPases and Rab11 family-interacting proteins (Rab11-FIPs) define integrated yet distinct compartments within the slow recycling pathway. The lipid content of these compartments is less well understood, although past studies have indicated phosphatidylserine (PS) is an integral component of recycling membranes. We sought to identify key differences in the presence of PS within Rab and Rab11-FIP containing membranes. We used live cell fluorescence microscopy and structured illumination microscopy to determine whether the previously published LactC2 probe for PS displays differential patterns of overlap with various Rab GTPases and Rab11-FIPs. Selective overlap was observed between the LactC2 probe and Rab GTPases when co-expressed in HeLa cells. Rab11-FIP1 proteins consistently overlapped with LactC2 along peripheral and pericentriolar compartments. The specificity of Rab11-FIP1 association with LactC2 was further confirmed by demonstrating that additional Rab11-FIPs (FIP2, FIP3, and FIP5) exhibited selective association with LactC2 containing compartments. Live cell dual expression studies of Rab11-FIPs with LactC2 indicated that PS is enriched along tubular compartments of the Rab11a-dependent recycling system. Additionally, we found that the removal of C2 domains from the Rab11-FIPs induced an accumulation of LactC2 probe in the pericentriolar region, suggesting that inhibition of trafficking through the recycling system can influence the distribution of PS within cells. Finally, we confirmed these findings using structured illumination microscopy suggesting that the overlapping fluorescent signals were on the same membranes. These results suggest distinct associations of Rab GTPases and Rab11-FIPs with PS-containing recycling system membrane domains.

Orchestration of cell surface proteins by Rab11

The organization of cells into interconnected structures such as animal tissues requires a sophisticated system directing receptors and adhesion proteins to the cell surface. The Rab11 small G proteins (Rab11a, b, and Rab25) of the Ras superfamily are master regulators of the surface expression of receptors and adhesion proteins. Acting as a molecular switch, Rab11 builds distinct molecular machinery such as motor protein complexes and the exocyst to transport proteins to the cell surface. Recent evidence reveals Rab11 localization at the trans-Golgi network (TGN), post-Golgi vesicles, and the recycling endosome, placing it at the intersection between the endocytic and exocytic trafficking pathways. We review Rab11 in various cellular contexts, and discuss its regulation and mechanisms by which Rab11 couples with effector proteins.

A functional interplay between the small GTPase Rab11a and mitochondria-shaping proteins regulates mitochondrial positioning and polarization of the actin cytoskeleton downstream of Src family kinases

It is believed that mitochondrial dynamics is coordinated with endosomal traffic rates during cytoskeletal remodeling, but the mechanisms involved are largely unknown. The adenovirus early region 4 ORF4 protein (E4orf4) subverts signaling by Src family kinases (SFK) to perturb cellular morphology, membrane traffic, and organellar dynamics and to trigger cell death. Using E4orf4 as a model, we uncovered a functional connection between mitochondria-shaping proteins and the small GTPase Rab11a, a key regulator of polarized transport via recycling endosomes. We found that E4orf4 induced dramatic changes in the morphology of mitochondria along with their mobilization at the vicinity of a polarized actin network typifying E4orf4 action, in a manner controlled by SFK and Rab11a. Mitochondrial remodeling was associated with increased proximity between Rab11a and mitochondrial membranes, changes in fusion-fission dynamics, and mitochondrial relocalization of the fission factor dynamin-related protein 1 (Drp1), which was regulated by the Rab11a effector protein FIP1/RCP. Knockdown of FIP1/RCP or inhibition of Drp1 markedly impaired mitochondrial remodeling and actin assembly, involving Rab11a-mediated mitochondrial dynamics in E4orf4-induced signaling. A similar mobilization of mitochondria near actin-rich structures was mediated by Rab11 and Drp1 in viral Src-transformed cells and contributed to the biogenesis of podosome rosettes. These findings suggest a role for Rab11a in the trafficking of Drp1 to mitochondria upon SFK activation and unravel a novel functional interplay between Rab11a and mitochondria during reshaping of the cell cytoskeleton, which would facilitate mitochondria redistribution near energy-requiring actin-rich structures.