Calbrain, a novel two EF-hand calcium-binding protein that suppresses Ca2+/calmodulin-dependent protein kinase II activity in the brain

A cDNA clone that encodes a novel Ca2+-binding protein was isolated from a human brain cDNA library. The gene for this clone, termed calbrain, encodes a 70-amino acid polypeptide with a predicted molecular mass of 8.06 kDa, T he analysis of deduced amino acid sequence revealed that calbrain contains two putative EF-hand motifs that show significantly high homology to those of the calmodulin (CaM) family rather than two EF-hand protein families. By Northern hybridization analysis, an approximate 1.5-kilobase pair transcri pt of calbrain was detected exclusively in the brain, and in situ hybridiza tion study revealed its abundant expression in the hippocampus, habenular a rea in the epithalamus, and in the cerebellum. A recombinant calbrain prote in showed a Ca2+ binding capacity, suggesting the functional potency as a r egulator of Ca2+-mediated cellular processes. Ca2+/calmodulin-dependent kin ase II, the most abundant protein kinase in the hippocampus and strongly im plicated in the basic neuronal functions, was used to evaluate the physiolo gical roles of calbrain, Studies in vitro revealed that calbrain competitiv ely inhibited CaM binding to Ca2+/calmodulin-dependent kinase II (K-i = 129 nM) and reduced its kinase activity and autophosphorylation.