Ca2+ Regulates Calmodulin Binding to IQ Motifs in IRS-1

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Abstract
IRS-proteins couple the receptors for insulin and various cytokines to signalling proteins containing Src homology 2 (SH2) domains. Here we demonstrate that calmodulin, a mediator of Ca2+-dependent physiological processes, associates with IRS-1 in a phosphotyrosine-independent manner. IRS-1 coimmunoprecipitated with calmodulin from lysates of Chinese hamster ovary cells expressing IRS-1. The interaction was modulated by Ca2+, and calmodulin binding to IRS-1 was enhanced by increasing intracellular Ca2+ with A23187. In contrast, trifluoperazine, a cell-permeable calmodulin antagonist, decreased binding of calmodulin to IRS-1. Insulin stimulated tyrosine phosphorylation of IRS-1, but did not significantly alter the interaction between calmodulin and IRS-1. IQ-like motifs occur between residues 106−126 and 839−859 of IRS-1. Synthetic peptides based on these sequences inhibited the association between IRS-1 and calmodulin. These data demonstrate that calmodulin binds to IRS-1 in intact cells in a Ca2+-regulated manner, providing a molecular link between the signalling pathways.