Who's Got the Calcium?

Abstract

The human cardiac voltage-gated sodium channel (hH1) is essential for normal heart function, and mutations in the alpha subunit of the channel cause deadly arrhythmias. Some of these mutations are in a region that confers critical regulation of channel function by calcium, but exactly how calcium regulates the channel has been unclear. The hH1 protein contains an EF-hand calcium-binding motif that may bind calcium directly and an IQ-type calmodulin-binding motif that could confer calcium-dependent changes through changes in calmodulin. Shah et al. explored interactions between these domains to help clarify the roles of these direct and indirect responses to calcium in channel function. The authors characterized the binding properties of various channel domains and mutants by monitoring 15N-3H heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectra. The EF-hand domain alone appeared to have a very low affinity for calcium (0.6 mM) that would be unlikely to have a physiological role. However, interaction of the IQ motif with the EF-hand domain enhanced calcium-binding affinity into the micromolar range. Contrary to expectations, a mutation in the EF-hand domain that causes arrhythmias in humans didn't alter calcium affinity but rather reduced affinity of the channel for binding calmodulin. On the other hand, a mutation in the IQ calmodulin-binding motif had little effect on affinity for calmodulin but reduced calcium-binding affinity. Also unusual was the finding that calcium-loaded calmodulin actually had lower affinity for an IQ motif-containing peptide than did calcium-free calmodulin. The authors propose that the effects of calcium can be explained as follows: In basal conditions, calmodulin is bound to the channel through the IQ motif. Increased concentrations of intracellular calcium weaken this interaction such that calmodulin is released, enabling the IQ motif to interact with the EF-hand domain, thus favoring calcium-dependent modulation of the channel through direct calcium binding. Further understanding of the intricate nature of these calcium-modulated domain interactions may reveal new therapeutic strategies to manage cardiac arrhythmias and other diseases associated with altered function of other members of this channel family in other cell types. V. N. Shah, T. L. Wingo, K. L. Weiss, C. K. Williams, J. R. Balser, W. J. Chazin, Calcium-dependent regulation of the voltage-gated sodium channel hH1: Intrinsic and extrinsic sensors use a common molecular switch. Proc. Natl. Acad. Sci. U.S.A. 103 , 3592-3597 (2006). [Abstract] [Full Text]