Cubilin (for mammalian systems, the vitamin B12–intrinsic factor receptor); BtuB (for E. coli and other Gram-negative bacteria, the outer membrane vitamin B12 receptor) (Cubn (for Cubilin in mammals); BtuB (in bacteria))

Cubilin is a large, extracellular, endocytic receptor highly expressed in the apical membrane of ileal enterocytes and kidney proximal tubules, where it binds the intrinsic factor–vitamin B12 complex and mediates its internalization in a calcium-dependent manner through interaction with amnionless. Loss of Cubilin function results in severe vitamin B12 deficiency, contributing to diseases like Imerslund-Gräsbeck syndrome. Cubilin is also a multi-ligand receptor involved in reabsorption of various proteins. BtuB is an outer membrane, TonB-dependent transporter in Escherichia coli and related bacteria, highly specific for the uptake of vitamin B12 from the environment. Mutations in BtuB can render it non-functional, which blocks vitamin B12 transport in bacteria.

Other names

Intrinsic factor-cobalamin receptorCUBAM receptor complexOuter membrane cobalamin transporter

For Cubilin: Receptor-mediated endocytosis of vitamin B12–intrinsic factor complex. For BtuB: Facilitated transport of cobalamin across the outer membrane using the TonB-dependent active transport system.