LD membrane protein cDNAs in pcDNA3.1+/C-(k)DYK were purchased from GenScript and their variants with the OPG2 tag, AUP1 point mutants and METTL7B/UBXD8 chimaeras were generated by site-directed mutagenesis. Sec61β with a C-terminal OPG2 tag in pcDNA5, either with or without an N-terminal FLAG epitope, has been described previously (Leznicki and High, 2020). Nuclease-treated rabbit reticulocyte lysate was obtained from Promega (L4960), [35S] methionine (EasyTag EXPRESS 35S Protein Labelling Mix) from PerkinElmer, LipidToxRed stain (H34476) from Thermo Fisher Scientific and EndoH (P0702 and P0703) from New England Biolabs. The hybridoma line producing the monoclonal anti-rhodopsin antibody (1:1000) (Adamus et al., 1991) was provided by Paul Hargrave (Department of Ophthalmology, University of Florida, USA), monoclonal anti-tubulin antibody (1:1000) by Keith Gull (University of Oxford, UK), rabbit polyclonal anti-Sec61α antibody (1:1000) was a gift from Richard Zimmermann and Sven Lang (Saarland University, Homburg, Germany), and rabbit anti-SRα antibodies (1:1000) (Jadhav et al., 2015) were a gift from Martin Pool (University of Manchester, UK). The following commercial antibodies were used: rabbit anti-β-actin (Abcam, ab8227, 1:5000, batch numbers GR3176830-1 and GR3224338-1); mouse anti-FLAG (clone M2, Sigma-Aldrich, F3165, 1:2500, batch number SLBH1191V); rabbit anti-FLAG (Sigma-Aldrich, F7425, 1:1000); anti-AUP1 (Bethyl Laboratories, A302-899A, 1:1000, batch number 1); anti-BAP31 (ProteinTech, 11200-1-AP, 1:1000, batch number 00018537); anti-ADRP (Abcam, ab78920, 1:1000, batch number GR3227109-1); anti-calnexin (Cell Signaling Technology, clone C5C9, 2679S, 1:2000, batch number 4); anti-MMGT1 (EMC5) (Bethyl Laboratories, A305-832A-M, 1:1000, batch number 1); anti-Pex3 (St John's Laboratory, STJ29491, 1:1000, batch number 949135460201); and anti-lamin B (SantaCruz Biotechnology, sc-6217, 1:1000, batch number J1311). MMGT1 (EMC5) siRNA was obtained from Thermo Fisher Scientific (s41129), whereas all the other siRNAs were made to order as ‘on-target+’ by Horizon Discovery. U2OS cells were procured from the European Collection of Authenticated Cell Cultures, whereas HeLa and HepG2 cells were obtained from the American Type Culture Collection, and all were checked for mycoplasma infection. Ipom-F was synthesised as described previously (Zong et al., 2015, 2020, 2017).