Towards a Structural Perspective of Calcium Regulation of L-Type Calcium Channels

Abstract

L-type Ca2+ channels are vital conduits for calcium influx in excitable cells, coupling electrical signals to downstream chemical signaling processes. Calmodulin (CaM) regulation of these channels comprises a feedback mechanism which alters channel activity in response to cytosolic Ca2+ elevations. This regulation takes the form of Ca2+ dependent inactivation (CDI), such that Ca2+ binding to either the N- or C-lobe of CaM induces a decrease in Ca2+ influx, whereby each lobe preferentially responds to either the global or local Ca2+ fluctuations. Mechanistically, this phenomenon depends upon the preassociation of Ca2+-free CaM (apoCaM) to the Ca2+ channel carboxy-terminus, which positions channels within a high open probability mode. That said, the atomistic details of both Ca2+/CaM and apoCaM interactions with respect to different channel elements remain largely unknown. NSCaTE (N-terminal Spatial Ca2+ Transforming Element) represents one such channel element, known to interact with Ca2+/CaM, resulting in a shift in the spatial decoding properties of the N-lobe of CaM (Nature 451:830). Here, we make use of a recent NMR structural model of NSCaTE interacting with the N-lobe of Ca2+/CaM (Front. Mol. Neurosci. 5:38) to gain insight into the structure-function aspects of this interaction. Through mutagenesis of key interacting residues, we have correlated this structural model with the strength of CaM regulation. Moreover, to obtain a structural understanding of apoCaM preassociation, we turned to recent crystallographic structure of the closely related NaV1.5 channel carboxy-terminus (Nat. Comms. 5:5126). Accordingly, we utilize homology modeling and molecular dynamics simulations to deduce a structural model for the interaction of individual lobes of apoCaM with the channel carboxy-terminus. Overall, this study elucidates the structural rearrangements underlying the modulation of individual lobes of CaM on L-type Ca2+ channel activity.