Calmodulin (CaM) regulation of CaV1-2 channels-termed calmodulation has proven rich, both biologically and as a general modulatory prototype with discriminating Ca2+ decoding capabilities. In earlier cycles, Ca2+-free CaM (apoCaM) was found to be pre-associated to an IQ domain on the intracellular carboxy terminus of channels. Ca2+-binding to this resident CaM induces as-yet-unclear conformational changes that somehow facilitate (CDF) or inactivate (CDI) channel opening, casting calmodulation as a positive or negative feedback control system for Ca2+. Intriguingly, Ca2+-binding to the C- and N-terminal lobes of CaM can each induce distinct forms of channel regulation, echoing earlier findings of CaM 'functional bipartition'in Paramecium. More remarkably, the C-lobe responds to the ~100-mM Ca2+ pulses driven by the associated channel (local Ca2+ selectivity), whereas the N-lobe is somehow capable of sensing far weaker signals from distant Ca2+ sources (global selectivity). In the current cycle, we deduced in coarse outline the functional mechanisms of calmodulation by the C-terminal lobe of CaM (slow CaM scheme), and by the N-terminal lobe (SQS scheme). These mechanisms explain the notable spatial Ca2+ decoding properties of these two calmodulatory subsystems. Prominently absent, however, is a quantitative sense of the Ca2+ dependence of these low-resolution mechanistic sketches, owing to the complexity of Ca2+ channel influx that normally drives calmodulation. A greater void concerns the channel/CaM interfaces postulated by slow CaM and SQS schemes. If we follow the lead of preliminary data, and abandon a prevailing 'IQ-centric'view, where apoCaM and Ca2+/CaM both interact with the IQ domain to trigger channel regulation, little would be known. This indeterminacy obscures the linkage of our promising functional mechanisms to molecular reality, and confounds understanding of just discovered physiological tuning of CaM regulation via RNA editing of CaV channels. These prominent challenges will be addressed by three specific aims. 1) To quantify the Ca2+ dependence of calmodulation via dynamic control of Ca2+ inputs to channels, as afforded by UV Ca2+ uncaging. 2) To identify the molecular states underlying CaM regulation of CaV channels, using an approach termed individually Transformed Langmuir (iTL) analysis, combined with in silico prediction of CaM/channel configurations. 3) To deduce, using Aim 2 insight, the mechanism whereby RNA editing of CaV1.3 channels alters their CaM regulation.
Calmodulin regulation of voltage-gated Ca channels is poised to impact diverse aspects of physiology, as well as maladies like cardiac arrhythmias, migraine, and neurodegenerative disease. Yet, serious gaps in our understanding of calmodulin regulation have stubbornly remained, owing to substantial complexity of and difficulty working with this regulatory system. Here, we will utilize new enabling technologies to shed light on these gaps, allowing us to harvest mechanistic advances for biological insight and eventual therapeutics.
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