Cubilin (intrinsic factor-vitamin B12 receptor) (Cubn)

Cubilin is a large, peripheral membrane receptor primarily expressed in the intestinal epithelium and kidney. It is responsible for the high-affinity, calcium- and cobalamin-dependent uptake of the intrinsic factor–vitamin B12 complex, crucial for efficient absorption of dietary vitamin B12. Human cubilin is composed of numerous CUB domains and epidermal growth factor repeats, with binding specificity for intrinsic factor–vitamin B12 mapped to specific CUB domains. Mutations or dysfunction in cubilin impair vitamin B12 absorption, leading to inherited forms of megaloblastic anemia such as Imerslund-Gräsbeck syndrome. Cubilin is not the same as "vitamin B12 activity"—rather, it is the molecular target responsible for mediating vitamin B12 activity in intestinal uptake. The term "vitamin B12 activity," as used here, is imprecise for a molecular target; it refers instead to the vitamin's cofactor roles, but the canonical receptor mediating B12 absorption is cubilin[2][5]. While vitamin B12 is a molecule with critical biological functions, "vitamin B12 activity" is not, itself, a *target*; the underlying physiologically relevant target for drug or molecular interaction in absorption is the cubilin receptor[2][5]. For most databases, you would not use "vitamin B12 activity" as a canonical target term—instead, it should be mapped to cubilin (IF-B12 receptor), or more specifically to vitamin B12–dependent enzymes (methionine synthase, methylmalonyl-CoA mutase) for intracellular B12 activity[3][4][7]. This makes "vitamin B12 activity" an incorrect target name.

Other names

Intrinsic factor-vitamin B12 receptorCUBNIF-B12 receptor

Facilitates endocytosis of the intrinsic factor–vitamin B12 complex into enterocytes for vitamin B12 absorption Dysfunction leads to failure of vitamin B12 uptake, causing deficiency