Calbindin

calbindin 1, 28kDa
NMR solution structure of Ca2+-loaded calbindin D28K.
Identifiers
Symbol	CALB1
Alt. symbols	CALB
Entrez	793
HUGO	1434
OMIM	114050
RefSeq	NM_004929
UniProt	P05937
Other data
Locus	Chr. 8 p11

Calbindin refers to several calcium-binding proteins. They were originally described as vitamin D-dependent calcium-binding proteins in the intestine and kidney in the chick and mammals. They are now classified in different sub-families as they differ in the number of Ca²⁺ binding EF-hand sites.

Calbindin-D28k was first shown to be present in the intestine in birds and then found in the mammalian kidney. It is also expressed in a number of neuronal and endocrine cells, particularly in the cerebellum. It is encoded in humans by the CALB1 gene.

Calbindin-D28k contains 4 active calcium-binding domains, and 2 modified domains that have lost their calcium-binding capacity. Calbindin-D28k acts as a calcium buffer and calcium sensor and can hold four Ca²⁺ in the EF-hands of loops EF1, EF3, EF4 and EF5. The structure was solved using high-resolution NMR and which was one of the largest proteins then to be determined. The sequence of Calbindin is 263 residues in length and has only one chain. The sequence consists mostly of alpha helicies but beta sheets are not absent. According to the PDB it is 44% helical with 14 helices containing 117 residues, and 4% beta sheet with 9 strands containing 13 residues.

Calbindin-D28k is a vitamin D responsive gene in many tissues, in particular the chick intestine, where it has a clear function in mediating calcium absorption. In the brain, its synthesis is independent of vitamin-D.

There is no homology between calbindin-D28k and calbindin-D9k, apart from their calcium binding domains (EF-hands): calbindin-D9k has two EF-hands, and calbindin-D28k has six.

Calretinin is a 29kDa protein with 58% homology to calbindin-D28k and principally found in nervous tissues. It is encoded in humans by the CALB2 gene.

Calbindin-D9k is present in mammalian intestinal epithelial cells (enterocytes). Calbindin-D9k can also be found in the kidney and uterus in some mammalian species. It in encoded in humans by the S100G gene which has also been termed CALB3.