Enzyme-inhibitor-like tuning of Ca2+ channel connectivity with calmodulin

Abstract

Ca2+ channels and calmodulin are two prominent signaling hubs1 that synergistically impact functions as diverse as cardiac excitability2, synaptic plasticity3, and gene transcription4. It is thereby fitting that these hubs are in some sense coordinated, as the opening of CaV1-2 Ca2+ channels are regulated by a single calmodulin (CaM) constitutively complexed with channels5. The Ca2+-free form of CaM (apoCaM) is already preassociated with the IQ domain on the channel carboxy terminus, and subsequent Ca2+ binding to this ‘resident’ CaM drives conformational changes that then trigger regulation of channel opening6. Another potential avenue for channel-CaM coordination could arise from the absence of Ca2+ regulation in channels lacking a preassociated CaM6,7. Natural fluctuations in CaM levels might then influence the fraction of regulatable channels, and thereby the overall strength of Ca2+ feedback. However, the prevailing view has been that the ultra-strong affinity of channels for apoCaM ensures their saturation with CaM8, yielding a significant form of concentration independence between Ca2+ channels and CaM. Here, we reveal significant exceptions to this autonomy, by combining electrophysiology to characterize channel regulation, with optical FRET sensor determination of free apoCaM concentration in live cells9. This approach translates quantitative CaM biochemistry from the traditional test-tube context, into the realm of functioning holochannels within intact cells. From this perspective, we find that long splice forms of CaV1.3 and CaV1.4 channels include a distal carboxy tail10-12 that resembles an enzyme competitive inhibitor, which retunes channel affinity for apoCaM so that natural CaM variations affect the strength of Ca2+ feedback modulation. Given the ubiquity of these channels13,14, the connection between ambient CaM levels and Ca2+ entry via channels is broadly significant for Ca2+ homeostasis. Strategies like ours promise key advances for the in situ analysis of signaling molecules resistant to in vitro reconstitution, such as Ca2+ channels.