Regulation of InsP3 receptor activity by neuronal Ca 2+-binding proteins

Inositol 1,4,5-trisphosphate receptors (InsP3Rs) were recently demonstrated to be activated independently of InsP3 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 3R-evoked Ca²⁺ signals. Using several experimental paradigms, including transient expression in COS cells, acute injection of recombinant protein into Xenopus oocytes and ⁴⁵Ca²⁺ flux from permeabilised COS cells, we demonstrated that CaBPs decrease the sensitivity of InsP3-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 InsP3R. This interaction resulted in decreased InsP3 binding to the receptor reminiscent of that observed for CaM. Unlike CaM, however, CaBPs do not inhibit ryanodine receptors, have a higher affinity for InsP3Rs 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 InsP3Rs tuning the sensitivity of cells to InsP3.