Regulatory interaction of sodium channel IQ-motif with calmodulin C-terminal lobe

An increasing number of ion channels have been found to be regulated by the direct binding of calmodulin (CaM), but its structural features are mostly unknown. Previously, we identified the Ca2+-dependent and -independent interactions of CaM to the voltage-gated sodium channel via an IQ-motif sequence. In this study we used the trypsin-digested CaM fragments (TR1C and TR2C) to analyze the binding of Ca2+-CaM or Ca2+-free (apo) CaM with a sodium channel-derived IQ-motif peptide (NaIQ). Circular dichroic spectra showed that NaIQ peptide enhanced alpha-helicity of the CaM C-terminal lobe, but not that of the CaM N-terminal lobe in the absence of Ca2+, whereas NaIQ enhanced the alpha-helicity of both the N- and C-terminal lobes in the presence of Ca2+. Furthermore, the competitive binding experiment demonstrated that Ca2+-dependent CaM binding of target peptides (MLCKp or melittin) with CaM was markedly suppressed by NaIQ. The results suggest that IQ-motif sequences contribute to prevent target proteins from activation at low Ca2+ concentrations and may explain a regulatory mechanism why highly Ca2+. sensitive target proteins are not activated in the cytoplasm. (C) 2003 Elsevier Inc. All rights reserved.