Regulation of InsP₃ receptor activity by neuronal Ca²⁺-binding proteins

Inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) were recently demonstrated to be activated independently of InsP(3) by a family of calmodulin (CaM)-like neuronal Ca2+-binding proteins (CaBPs). We investigated the interaction of both naturally occurring long and short CaBP1 isoforms with InsP3Rs, and their functional effects on InsP(3)R-evoked Ca2+ signals. Using several experimental paradigms, including transient expression in COS cells, acute injection of recombinant protein into Xenopus oocytes and Ca-45(2+) flux from permeabilised COS cells, we demonstrated that CaBPs decrease the sensitivity of InsP(3)-induced Ca2+ release (IICR). In addition, we found a Ca2+-independent interaction between CaBP1 and the NH2-terminal 159 amino acids of the type 1 InsP(3)R. This interaction resulted in decreased InsP(3) binding to the receptor reminiscent of that observed for CaM. Unlike CaM, however, CaBPs do not inhibit ryanodine receptors, have a higher affinity for InsP(3)Rs and more potently inhibited IICR. We also show that phosphorylation of CaBP1 at a casein kinase 2 consensus site regulates its inhibition of IICR. Our data suggest that CaBPs are endogenous regulators of InsP(3)Rs tuning the sensitivity of cells to InsP(3).