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The N-terminal Ca²⁺-Independent Calmodulin-Binding Site on the Inositol 1,4,5-trisphosphate Receptor Is Responsible for Calmodulin Inhibition, Even Though This Inhibition Requires Ca²⁺

Laboratorium voor Fysiologie, Katholieke Universiteit Leuven Campus Gasthuisberg O/N, Leuven, Belgium (N.N.K., G.B., K.S., J.B.P., G.C., L.M., H.d.S.); Molecular and Cellular Biology Program and Department of Veterinary and Animal Sciences, University of Massachusetts, Amherst, Massachusetts (J.S., R.A.F.); and Department of Molecular Neurobiology, Institute of Medical Science, University of Tokyo, Tokyo, Japan (K.M.)

Calmodulin (CaM) is a ubiquitous Ca²⁺-sensor protein that plays an important role in regulating a large number of Ca²⁺ channels, including the inositol 1,4,5-trisphosphate receptor (IP₃R). CaM binds to the IP₃R at Ca²⁺-dependent as well as at Ca²⁺-independent interaction sites. In this study, we have investigated the Ca²⁺-independent CaM-binding site for its role in the regulation of the Ca²⁺-dependent bell-shaped activation curve of the IP₃R. Suramin, a polysulfonated napthylurea, displaced CaM in both the presence and the absence of Ca²⁺. Suramin competed with CaM for binding to different peptides representing the previously identified CaM-binding sites on IP₃R1. By interacting with the N-terminal Ca²⁺-independent CaM-binding site, suramin mimicked the functional effect of CaM and induced an allosteric but competitive inhibition of IP₃ binding. Therefore, suramin also potently inhibited IP₃-induced Ca²⁺ release (IICR) from permeabilized cells predominantly expressing IP₃R1 (L15 fibroblasts) or IP₃R3 (Lvec fibroblasts), even though the IP₃R3 does not contain Ca²⁺-dependent CaM-binding sites. Furthermore, we have found that CaM₁₂₃₄, a CaM mutated in its four EF hands, inhibited IICR in a Ca²⁺-dependent way with the same potency as CaM. We conclude that CaM inhibits IICR via the N-terminal binding site. The inhibition requires Ca²⁺ but CaM itself is not the Ca²⁺ sensor for the inhibition of the IP₃R.

Received February 25, 2004; accepted April 18, 2004.

Address correspondence to: Humbert De Smedt, Laboratorium voor Fysiologie, K. U. Leuven Campus Gasthuisberg O/N, Herestraat 49, B-3000, Leuven, Belgium. E-mail: humbert.desmedt{at}med.kuleuven.ac.be

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