Mechanism of cargo recognition by retromer-linked SNX-BAR proteins

Abstract

Endocytic recycling of internalized transmembrane proteins is essential for many important physiological processes. Recent studies have revealed that retromer-related Sorting Nexin family (SNX)-Bin/Amphiphysin/Rvs (BAR) proteins can directly recognize cargoes like cation-independent mannose 6-phosphate receptor (CI-MPR) and Insulin-like growth factor 1 receptor (IGF1R); however, it remains poorly understood how SNX-BARs select specific cargo proteins and whether they recognize additional ligands. Here, we discovered that the binding between SNX-BARs and CI-MPR or IGF1R is mediated by the phox-homology (PX) domain of SNX5 or SNX6 and a bipartite motif, termed SNX-BAR-binding motif (SBM), in the cargoes. Using this motif, we identified over 70 putative SNX-BAR ligands, many of which play critical roles in apoptosis, cell adhesion, signal transduction, or metabolite homeostasis. Remarkably, SNX-BARs could cooperate with both SNX27 and retromer in the recycling of ligands encompassing the SBM, PDZ-binding motif, or both motifs. Overall, our studies establish that SNX-BARs function as a direct cargo-selecting module for a large set of transmembrane proteins transiting the endosome, in addition to their roles in phospholipid recognition and biogenesis of tubular structures.have been linked with a variety of human diseases including Alzheimer's disease, Parkinson's disease, cancer, and diabetes [4,5].Key protein machineries important for the sequence-dependent recycling include the evolutionarily conserved retromer complex (vacuolar protein sorting 35 [VPS35]/VPS26/VPS29 in higher eukaryotes) [6][7][8], the recently discovered retriever [9], the WASH complex [10][11][12], and members of the Sorting Nexin family (SNX) [13][14][15]. A subset of SNX proteins possessing a Bin/Amphiphysin/Rvs (BAR) domain, in addition to the phox-homology (PX) domain, have been linked with the retromer complex. The retromer-related SNX-BAR proteins (referred as SNX-BARs herein) SNX1, SNX2, SNX5, SNX6, and SNX32 form heterodimeric complexes and are critical for both endosome-to-plasma membrane recycling and endosome-to-TGN retrieval [16,17]. Current models suggest that SNX-BARs promote the endosome-to-plasma membrane recycling via associating with SNX27 and retromer, with the PDZ domain of SNX27 as the predominant cargo-recognition module [14]. For endosome-to-TGN trafficking, one of the best-characterized cargoes is cation-independent mannose 6-phosphate receptor (CI-MPR), which is necessary to deliver newly synthesized lysosomal hydrolases to the endosomal lumen and thus critical for lysosomal function [18]. However, previous studies have provided contradictory models regarding the role of retromer and SNX-BARs in the endosome-to-TGN retrieval of CI-MPR. Work from many different labs has initially supported the idea that retromer is necessary for the retrieval of CI-MPR, likely through a direct association with its cytoplasmic tail, in particular, the hydrophobic WLM motif [19][20][21]. However, recent work by Cullen and Steinberg has provi...

SNX-PXA-RGS-PXC Subfamily of SNXs in the Regulation of Receptor-Mediated Signaling and Membrane Trafficking

“…The weak association of SNX-BAR with the retromer in mammalian cells may reflect the large diversity of cargoes and the need for other proteins, such as Rab GTPases [ 66 , 67 ], ubiquitin [ 68 ], actin filaments [ 67 ], and WASH complex [ 69 ], to coordinate in the regulation of receptor signaling and trafficking [ 3 , 7 ]. The retromer is also critical for SNX-BAR regulation of receptor endocytic trafficking, retromer-independent receptor plasma membrane recycling, and endosome-to-TGN retrograde trafficking [ 61 , 70 , 71 ]. Because the SNX-PXA-RGS-PXC subfamily lacks the BAR domain, it does not depend on the retromer or other retrieval machineries to regulate receptor cargo signaling and trafficking.…”

Retromer dysfunction at the nexus of tauopathies

“…These trafficking itineraries are critical for the maintenance of receptor homeostasis and therefore downstream functions, otherwise these receptors are defaulted to the lysosome for destruction [117]. Selection of receptors at the endosomal membrane is a tightly regulated process with different receptors selected by distinct surfaces of the retromer complex [118][119][120]. Engagement with specific receptors can also be facilitated by subcomplexes of retromer with itinerary-specific adapters [118].…”

Sorting Nexins in Protein Homeostasis

“…Current models propose that both PX and BAR domains have to be engaged with the membrane to ensure specificity and efficient binding. Mammalian cells possess twelve SNX-BAR family members (SNX1, SNX2, SNX4, SNX5, SNX7, SNX8, SNX9, SNX18, SNX30, SNX32, and SNX33) [ 58 ]. Many SNX-BAR proteins (SNX1, SNX2, SNX5, SNX6, and the neuronal SNX32) form heterodimeric complexes.…”

“…Supporting these studies, Simonetti et al used a SILAC-based proteomic approach to identify a recycling motif, (WLM) [ 65 ]. Yong et al extended this work and both identified an additional upstream hydrophobic stretch to the recycling motif and over 70 putative SNX-BAR cargos [ 58 ]. Based upon these results, an alternative model has been proposed in which SNX-BARs function as a direct cargo selecting module for a large set of transmembrane proteins transiting the endosome.…”