PGRMC1 acts as a size-selective cargo receptor to drive ER-phagic clearance of mutant prohormones

Abstract

The reticulon-3 (RTN3)-driven targeting complex promotes clearance of misfolded prohormones from the endoplasmic reticulum (ER) for lysosomal destruction by ER-phagy. Because RTN3 resides in the cytosolic leaflet of the ER bilayer, the mechanism of selecting misfolded prohormones as ER-phagy cargo on the luminal side of the ER membrane remains unknown. Here we identify the ER transmembrane protein PGRMC1 as an RTN3-binding partner. Via its luminal domain, PGRMC1 captures misfolded prohormones, targeting them for RTN3-dependent ER-phagy. PGRMC1 selects cargos that are smaller than the large size of other reported ER-phagy substrates. Cargos for PGRMC1 include mutant proinsulins that block secretion of wildtype proinsulin through dominant-negative interactions within the ER, causing insulin-deficiency. Chemical perturbation of PGRMC1 partially restores WT insulin storage by preventing ER-phagic degradation of WT and mutant proinsulin. Thus, PGRMC1 acts as a size-selective cargo receptor during RTN3-dependent ER-phagy, and is a potential therapeutic target for diabetes.

ER-phagy: mechanisms, regulation, and diseases connected to the lysosomal clearance of the endoplasmic reticulum

“…Associations between TEX264, CNX, and the ER membrane-bound oxidoreductase TMX1 and of plant Rtn1 and Rtn2 with CNX, BiP, and PDI have also been reported, but their relevance in selecting cargo and/or delimiting the ER subdomains to be removed remains to be understood ( 238 , 248 , 306 ). Moreover, the transmembrane ER protein Progesterone Receptor Membrane Component 1 (PGRMC1) interacts with RTN3 proteins and mediates the clearance of ER subdomains containing a subset of misfolded clients during RTN3L-mediated ER-phagy ( 307 ) (see also sect. 3.3.2.2.2.4).…”

“…The finding that the ER-to-lysosome-associated degradation defect caused by the depletion of RTN3 proteins is rescued upon backtransfection of either the short form of RTN3 or RTN4, both of which lack LIR motifs, warrants further investigation ( 440 ). In this RTN3-driven ER-to-lysosome-associated degradation, the transmembrane ER protein PGRMC1, which in contrast to RTN3 proteins has a luminal domain, recruits the clients to be segregated in ER subdomains that are eventually delivered to the degradative compartments for clearance ( 307 ). An alternative model suggests a role of RTN3L as an ER-phagy receptor in these catabolic pathways, which rely on the segregation of the ER-to-lysosome-associated degradation clients in ER-phagy sites also displaying the COPII coat subunit SEC24C ( 457 ).…”

Time-Resolved Interactome Profiling Deconvolutes Secretory Protein Quality Control Dynamics

“…HERPUD1 is a ubiquitin-like protein and associates with VCP during ERAD (Christianson et al, 2012; Needham et al, 2019; Okuda-Shimizu and Hendershot, 2007). The ER-phagy receptors RTN3 and TEX264 localize to sub domains of the ER to facilitate degradation of specific ER clients and organellular regions (An et al, 2019; Chen et al, 2021; Chino et al, 2019; Cunningham et al, 2019; Fielden et al, 2022; Grumati et al, 2017). Of note, while the ER-phagy receptor CCPG1 was identified in our mass spectrometry dataset siRNA silencing of CCPG1 did not significantly alter Tg-NLuc abundance in lysate or media, nor did silencing of SEC62 or RETREG1 (FAM134B), two additional ER-phagy receptors found to regulate ER dynamics (Figure S5; Table S6).…”

ER ‐phagy: selective autophagy of the endoplasmic reticulum

“…Since RTN3, as well as FAM134B, does not possess an ER lumenal region, another protein that directly recognized these mutant prohormones in the ER lumen was assumed to cooperate with RTN3. Indeed, a proteomic approach in a continuing work identified the single‐spanning membrane protein PGRMC1 as an RTN3 interactor (Chen et al , 2021 ). Knockdown of PGRMC1, as well as that of RTN3, resulted in the accumulation of aggregates of mutant prohormones in the ER lumen.…”

“…Proinsulin mutants entangle the wild‐type protein and thereby inhibit its secretion in a dominant‐negative manner. RTN3 overexpression and PGRMC1 knockdown were both shown to increase the secretion of wild‐type proinsulin in the presence of mutant proinsulin, providing a new insight into the development of treatments for MIDY (Cunningham et al , 2019 ; Chen et al , 2021 ).…”