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...

Lipid raft is an important element for the cellular entry of some viruses, including coronavirus infectious bronchitis virus (IBV). However, the exact role of lipid rafts in the cellular membrane during the entry of IBV into host cells is still unknown. In this study, we biochemically fractionated IBV-infected cells via sucrose density gradient centrifugation after depleting plasma membrane cholesterol with methyl-β-cyclodextrin or Mevastatin. Our results demonstrated that unlike IBV non-structural proteins, IBV structural proteins co-localized with lipid raft marker caveolin-1. Infectivity assay results of Vero cells illustrated that the drug-induced disruption of lipid rafts significantly suppressed IBV infection. Further studies revealed that lipid rafts were not required for IBV genome replication or virion release at later stages. However, the drug-mediated depletion of lipid rafts in Vero cells before IBV attachment significantly reduced the expression of viral structural proteins, suggesting that drug treatment impaired the attachment of IBV to the cell surface. Our results indicated that lipid rafts serve as attachment factors during the early stages of IBV infection, especially during the attachment stage.

Endocytosed proteins can be delivered to lysosomes for degradation or recycled to either the trans-Golgi network or the plasma membrane. It remains poorly understood how the recycling versus degradation of cargoes is determined. Here, we show that multiple extracellular stimuli, including starvation, LPS, IL-6, and EGF treatment, can strongly inhibit endocytic recycling of multiple cargoes through the activation of MAPK11/14. The stress-induced kinases in turn directly phosphorylate SNX27, a key regulator of endocytic recycling, at serine 51 (Ser51). Phosphorylation of SNX27 at Ser51 alters the conformation of its cargo-binding pocket and decreases the interaction between SNX27 and cargo proteins, thereby inhibiting endocytic recycling. Our study indicates that endocytic recycling is highly dynamic and can crosstalk with cellular stress–signaling pathways. Suppression of endocytic recycling and enhancement of receptor lysosomal degradation serve as new mechanisms for cells to cope with stress and save energy.

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