Unique calcium-dependent hydrophobic binding proteins: possible independent mediators of intracellular calcium distinct from calmodulin.

Calcium-dependent regulation of cellular processes is mediated by specific intracellular proteins. A newly described set of proteins isolated from chicken gizzard with Mr of 67 X 10(3), 35 X 10(3), 33 X 10(3) and 30 X 10(3) also express a hydrophobic site in the presence of calcium. These proteins are isolated from several other cellular tissues and are termed calcimedins. These proteins differ from calmodulin in isoelectric point, DEAE-cellulose binding characteristics and heat stability. The calcimedins do not activate calmodulin-dependent cyclic nucleotide phosphodiesterase but do activate a hepatic microsomal Ca2+ -ATPase system. Hence, the possibility is opened that calcium regulation of cellular processes is mediated by calcium-binding proteins in addition to calmodulin.