which has also been termed CALB3. Nonetheless, there is no homology between calbindin 1 and S100G, apart from their calcium binding domains (EF-hands): S100G has two EF-hands, and calbindin 1 has six. Unlike calbindin 1 and 2, S100G is a member of the S100 family of calcium-binding proteins. 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. 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. 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. In mice in which the vitamin D receptor is not expressed, S100G is less abundant, but not absent. Discovery Vitamin D-dependent calcium binding proteins were discovered in the cytosolic fractions of chicken intestine, and later in mammalian intestine and kidney, by researchers including Robert Wasserman of Cornell University. Such proteins bound calcium in the micromolar range and were greatly reduced in vitamin D-deficient animals. Expression could be induced by treating these animals with vitamin D metabolites such as calcitriol. They were found to exist in two distinct sizes with a molecular weight of approximately 9 kDa and 28 kDa, and they were renamed calbindins. References Protein families