Transcobalamin receptor CD320, responsible for vitamin B12 cellular uptake, is present on the cell surface as a homo-oligomer

CD320, also known as the transcobalamin receptor, is a key receptor that mediates the cellular uptake of circulating vitamin B₁₂ bound to transcobalamin. In humans, CD320 abnormalities cause metabolic and neurological disorders. Previous biochemical studies indicate that the CD320 is a monomeric receptor. In this study, we conducted molecular and cellular experiments in human embryonic kidney 293 cells to examine biosynthesis and molecular forms of human CD320. We found that CD320 was present on the cell surface in an oligomeric form, which consisted of five homomers interconnected via disulfide bonds, possibly in one or both low-density lipoprotein receptor (LDLR)-like domains. Deletion of the LDLR-like domains prevented CD320 oligomerization and cell surface localization, thereby impairing cellular uptake of vitamin B₁₂. By analyzing biochemical forms of CD320 in intracellular compartments, we showed that CD320 oligomerization occurred in the endoplasmic reticulum (ER) and that this process required the transmembrane domain but not N- or O-glycosylation of CD320. Moreover, we showed that the CD320 ΔE88 variant, identified in infants with abnormal vitamin B₁₂ metabolism, did not prevent CD320 oligomerization but delayed CD320 trafficking out of the ER, resulting in low levels of CD320 oligomers on the cell surface. Together, our findings provide important insights into the biochemical nature and cellular mechanisms underlying the function of CD320 and associated pathologies in vitamin B₁₂ metabolism.