A NOVEL ROLE FOR CALMODULIN - CA2-INDEPENDENT INHIBITION OF TYPE-1 INOSITOL TRISPHOSPHATE RECEPTORS()

Calmodulin inhibits both inositol 1,4,5-trisphosphate (IP3) binding to , and IP3-evoked Ca2+ release by, cerebellar IP3 receptors [Patel, Mor ris, Adkins, O' Beirne and Taylor (1997) Proc. Natl. Acad. Sci. U.S.A, 94, 11627-11632]. In the present study, full-length rat type-1 and -3 IP3 receptors were expressed at high levels in insect Spodoptera frug iperda 9 cells and the effects of calmodulin were examined. In the abs ence of Ca2+, calmodulin caused a concentration-dependent and reversib le inhibition of [H-3]IP3 binding to type-1 IP3 receptors by decreasin g their apparent affinity for IP3. The effect was not reproduced by hi gh concentrations of troponin C, parvalbumin or S-100. Increasing the medium free [Ca2+] ([Ca2+](m)) inhibited [H-3]IP3 binding to type-1 re ceptors, but the further inhibition caused by a submaximal concentrati on of calmodulin was similar at each [Ca2+](m). In the absence of Ca2, I-125-calmodulin bound to a single site on each type-1 receptor subu nit and to an additional site in the presence of Ca2+. There was no de tectable binding of I-125-calmodulin to type-3 receptors and binding o f [H-3]IP3 was insensitive to calmodulin at all [Ca2+](m). Both peptid e and conventional Ca2+-calmodulin antagonists affected neither [H-3]I P3 binding directly nor the inhibitory effect of calmodulin in the abs ence of Ca2+, but each caused a [Ca2+](m)-dependent reversal of the in hibition of [H-3]IP3, binding caused by calmodulin. Camstatin, a pepti de that binds to calmodulin equally well in the presence or absence of Ca2+, reversed the inhibitory effects of calmodulin on [H-3]IP3 bindi ng at all [Ca2+](m). We conclude that calmodulin specifically inhibits [H-3]IP3 binding to type-1 IP3 receptors: the first example of a prot ein regulated by calmodulin in an entirely Ca2+-independent manner. In hibition of type-1 IP3 receptors by calmodulin may dynamically regulat e their sensitivity to IP3 in response to the changes in cytosolic fre e calmodulin concentration thought to accompany stimulation of neurone s.