The endoplasmic reticulum (ER)-resident intramembrane rhomboid protease RHBDL4 generates metastable protein fragments and together with the ER-associated degradation (ERAD) machinery provides a clearance mechanism for aberrant and surplus proteins. However, the endogenous substrate spectrum and with that the role of RHBDL4 in physiological ERAD is mainly unknown. Here, we use a substrate trapping approach in combination with quantitative proteomics to identify physiological RHBDL4 substrates. This revealed oligosaccharyltransferase (OST) complex subunits such as the catalytic active subunit STT3A as substrates for the RHBDL4dependent ERAD pathway. RHBDL4-catalysed cleavage inactivates OST subunits by triggering dislocation into the cytoplasm and subsequent proteasomal degradation. RHBDL4 thereby controls the abundance and activity of OST, suggesting a novel link between the ERAD machinery and glycosylation tuning.

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Protein degradation is fundamentally important to ensure cell homeostasis. In the Endoplasmic Reticulum (ER), the ER-associated degradation (ERAD) pathway targets incorrectly folded and unassembled proteins into the cytoplasm for turnover by the proteasome. In contrast, lysosomal degradation serves as failsafe mechanism for removal of proteins that resist ERAD by forming aggregates. In previous work, we showed that the ERresident rhomboid protease RHBDL4, together with p97, mediates membrane protein degradation. However, whether RHBDL4 acts in concert with additional ERAD components is unclear and its full substrate spectrum remains to be defined. Here, we show that besides membrane proteins, RHBDL4 cleaves aggregation-prone, luminal ERAD substrates including a soluble version of the major histocompatibility complex heavy chain (MHC202). RHBDL4's interaction with erlin ERAD substrate receptors and reciprocal interaction of MHC202 with erlins suggest that RHBDL4 defines a substrate clipping mechanism that rescues aggregation-prone peptides in the ER lumen from terminal aggregation.Abbreviations ER, endoplasmic reticulum; ERAD, ER-associated degradation; MHC, major histocompatibility complex; TM, transmembrane; UPR, unfolded protein response.