The function of TUSC3 (tumor suppressor candidate 3, Ensembl gene identifier ENSG00000104723) is as follows. Acts as accessory component of the N-oligosaccharyl transferase (OST) complex which catalyzes the transfer of a high mannose oligosaccharide from a lipid-linked oligosaccharide donor to an asparagine residue within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains. Involved in N-glycosylation of STT3B-dependent substrates. Specifically required for the glycosylation of a subset of acceptor sites that are near cysteine residues; in this function seems to act redundantly with MAGT1. In its oxidized form proposed to form transient mixed disulfides with a glycoprotein substrate to facilitate access of STT3B to the unmodified acceptor site. Also has oxidoreductase-independent functions in the STT3B-containing OST complex possibly involving substrate recognition. Could indirectly play a role in Mg(2+) transport (PubMed:19717468). {ECO:0000269|PubMed:19717468, ECO:0000269|PubMed:25135935, ECO:0000305|PubMed:12887896, ECO:0000305|PubMed:24685145}.