Calbindin
calbindin 1, 28kDa | |||||||
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Chr. 8 p11 | |||||||
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calbindin 2, 29kDa (calretinin) | |||||||
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Identifiers | |||||||
Symbol | CALB2 | ||||||
Chr. 16 q22.1 | |||||||
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Calbindins are three different calcium-binding proteins: calbindin, calretinin and S100G. They were originally described as vitamin D-dependent calcium-binding proteins in the intestine and kidney of chicks and mammals. They are now classified in different subfamilies as they differ in the number of Ca2+ binding EF hands.
Calbindin 1
Calbindin 1 or simply calbindin 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 a 28 kDa protein encoded in humans by the CALB1 gene.
Calbindin contains 4 active calcium-binding
Calbindin is a vitamin D–responsive gene in many tissues, in particular the chick intestine, where it has a clear function in mediating calcium absorption.[5] In the brain, its synthesis is independent of vitamin-D.
Calbindin 2 (Calretinin)
Calretinin, also known as calbindin 2, is a 29 kDa protein with 58% homology to calbindin 1 and principally found in nervous tissues.[6] It is encoded in humans by the CALB2 gene and was formerly known as calbindin-D29k.
Calbindin 3 (S100G)
S100G, formerly calbindin 3 and calbindin-D9k, is present in mammalian
S100G mediates the transport of calcium across the enterocytes from the apical side, where entry is regulated by the calcium channel TRPV6, to the basolateral side, where calcium pumps such as PMCA1 utilize intracellular adenosine triphosphate to pump calcium into the blood.[7] The transport of calcium across the enterocyte cytoplasm appears to be rate-limiting for calcium absorption in the intestine; the presence of calbindin increases the amount of calcium crossing the cell without raising the free concentration.[8] S100G may also stimulate the basolateral calcium-pumping ATPases. Expression of S100G, like that of calbindin 1, is stimulated by the active vitamin D metabolite, calcitriol although the precise mechanisms are still controversial.[9] In mice in which the vitamin D receptor is not expressed, S100G is less abundant, but not absent.[citation needed]
Discovery
Vitamin D-dependent calcium binding proteins were discovered in the cytosolic fractions of chicken
They were found to exist in two distinct sizes with a
References
This article incorporates text from the United States National Library of Medicine, which is in the public domain.