Powerful and Ancient Embrace of Four-Domain Voltage-Gated Channels with Calmodulin

Abstract

A most ubiquitous Ca2+-sensing molecule throughout biology---calmodulin (CaM)---serves as a virtual subunit of numerous ion channels, conferring vital Ca2+-dependent modulation of channel opening. Nowhere is this Ca2+ modulation more prevalent than in voltage-gated Ca2+ channels, where CaM dynamically switches among differing interactions with a proximal carboxyl tail region (CI domain), thus translating Ca2+ fluctuations into profound adjustments in channel opening. Using new approaches, we here uncover striking further actions of CaM. Using chemical-biological tools to step increase CaM concentration, we reveal that the binding of Ca2+-free CaM (apoCaM) to channels itself imparts a striking boost in baseline open probability, with ramifications for Ca2+ homeostasis. Adopting a synthetic-biological approach of radical reductionism, we demonstrate that localized CaM predominates in enabling channel expression: even when the entire carboxy tail of a channel is excised, robust Ca2+ currents are nonetheless sustained if CaM is tethered to these ‘reduced’ channels. Finally, using photouncaging to produce measured step increases of Ca2+, we now recognize that the CaM/CI regulatory module extends to the superfamily of Na channels, another key member of four-domain channels. Though Ca2+ and Na channels have generally been considered distinct, their carboxy tails exhibit tantalizing homology in the region spanning the CI domain. However, a decade of Na channel research has revealed only subtle and variable Ca2+ effects, with divergent mechanisms. Photouncaging of Ca2+ demonstrates here that the dissimilarities in Na channels are only apparent, and that function and mechanism are fundamentally conserved to an astonishing degree across Ca2+ and Na channels. Given the common heritage of these channels dating to the early days of eukaryotes, the present results link modern-day CI elements to a legitimately primeval modulatory design, and cast CaM as a partner in ion-channel regulation throughout much of living history.