Component of the multi-pass translocon (MPT) complex that mediates insertion of multi-pass membrane proteins into the lipid bilayer of membranes (PubMed:36261522). The MPT complex takes over after the SEC61 complex: following membrane insertion of the first few transmembrane segments of proteins by the SEC61 complex, the MPT complex occludes the lateral gate of the SEC61 complex to promote insertion of subsequent transmembrane regions (PubMed:36261522). Within the MPT complex, the GEL subcomplex may mediate insertion of transmembrane regions into the membrane (PubMed:36261522). In addition to its role in multi-pass membrane insertion, RAB5IF/OPTI also acts as an assembly factor for mitochondrial respiratory complexes (PubMed:31536960). {ECO:0000269|PubMed:31536960, ECO:0000269|PubMed:36261522}. This is the function of RAB5IF (RAB5 interacting factor, Ensembl gene identifier ENSG00000101084).