Conservation of Ca2+/calmodulin regulation across Na and Ca2+ channels.

Voltage-gated Na and Ca2+ channels comprise distinct ion channel superfamilies, yet the carboxy tails of these channels exhibit high homology, hinting at a long-shared and purposeful module. For different Ca2+ channels, carboxyl-tail interactions with calmodulin do elaborate robust and similar forms of Ca2+ regulation. However, Na channels have only shown subtler Ca2+ modulation that differs among reports, challenging attempts at unified understanding. Here, by rapid Ca2+ photorelease onto Na channels, we reset this view of Na channel regulation. For cardiac-muscle channels (NaV1.5), reported effects from which most mechanistic proposals derive, we observe no Ca2+ modulation. Conversely, for skeletal-muscle channels (NaV1.4), we uncover fast Ca2+ regulation eerily similar to that of Ca2+ channels. Channelopathic myotonia mutations halve NaV1.4 Ca2+ regulation, and transplanting the NaV1.4 carboxy tail onto Ca2+ channels recapitulates Ca2+ regulation. Thus, we argue for the persistence and physiological relevance of an ancient Ca2+ regulatory module across Na and Ca2+ channels.

CaV channels reject signaling from a second CaM in eliciting Ca2+-dependent feedback regulation.

Calmodulin (CaM) regulation of voltage-gated calcium (CaV1-2) channels is a powerful Ca2+-feedback mechanism to adjust channel activity in response to Ca2+ influx. Despite progress in resolving mechanisms of CaM-CaV feedback, the stoichiometry of CaM interaction with CaV channels remains ambiguous. Functional studies that tethered CaM to CaV1.2 suggested that a single CaM sufficed for Ca2+ feedback, yet biochemical, FRET, and structural studies showed that multiple CaM molecules interact with distinct interfaces within channel cytosolic segments, suggesting that functional Ca2+ regulation may be more nuanced. Resolving this ambiguity is critical as CaM is enriched in subcellular domains where CaV channels reside, such as the cardiac dyad. We here localized multiple CaMs to the CaV nanodomain by tethering either WT or mutant CaM that lack Ca2+-binding capacity to the pore-forming α-subunit of CaV1.2, CaV1.3, and CaV2.1 and/or the auxiliary ß2A subunit. We observed that a single CaM tethered to either the α or ß2A subunit tunes Ca2+ regulation of CaV channels. However, when multiple CaMs are localized concurrently, CaV channels preferentially respond to signaling from the α-subunit-tethered CaM. Mechanistically, the introduction of a second IQ domain to the CaV1.3 carboxyl tail switched the apparent functional stoichiometry, permitting two CaMs to mediate functional regulation. In all, Ca2+ feedback of CaV channels depends exquisitely on a single CaM preassociated with the α-subunit carboxyl tail. Additional CaMs that colocalize with the channel complex are unable to trigger Ca2+-dependent feedback of channel gating but may support alternate regulatory functions.

Allostery in Ca²âº channel modulation by calcium-binding proteins.

Distinguishing between allostery and competition among modulating ligands is challenging for large target molecules. Out of practical necessity, inferences are often drawn from in vitro assays on target fragments, but such inferences may belie actual mechanisms. One key example of such ambiguity concerns calcium-binding proteins (CaBPs) that tune signaling molecules regulated by calmodulin (CaM). As CaBPs resemble CaM, CaBPs are believed to competitively replace CaM on targets. Yet, brain CaM expression far surpasses that of CaBPs, raising questions as to whether CaBPs can exert appreciable biological actions. Here, we devise a live-cell, holomolecule approach that reveals an allosteric mechanism for calcium channels whose CaM-mediated inactivation is eliminated by CaBP4. Our strategy is to covalently link CaM and/or CaBP to holochannels, enabling live-cell fluorescence resonance energy transfer assays to resolve a cyclical allosteric binding scheme for CaM and CaBP4 to channels, thus explaining how trace CaBPs prevail. This approach may apply generally for discerning allostery in live cells.

Enzyme-inhibitor-like tuning of Ca(2+) channel connectivity with calmodulin.

Ca(2+) channels and calmodulin (CaM) are two prominent signalling hubs that synergistically affect functions as diverse as cardiac excitability, synaptic plasticity and gene transcription. It is therefore fitting that these hubs are in some sense coordinated, as the opening of Ca(V)1-2 Ca(2+) channels are regulated by a single CaM constitutively complexed with channels. The Ca(2+)-free form of CaM (apoCaM) is already pre-associated with the isoleucine-glutamine (IQ) domain on the channel carboxy terminus, and subsequent Ca(2+) binding to this 'resident' CaM drives conformational changes that then trigger regulation of channel opening. Another potential avenue for channel-CaM coordination could arise from the absence of Ca(2+) regulation in channels lacking a pre-associated CaM. Natural fluctuations in CaM concentrations might then influence the fraction of regulable channels and, thereby, the overall strength of Ca(2+) feedback. However, the prevailing view has been that the ultrastrong affinity of channels for apoCaM ensures their saturation with CaM, yielding a significant form of concentration independence between Ca(2+) channels and CaM. Here we show that significant exceptions to this autonomy exist, by combining electrophysiology (to characterize channel regulation) with optical fluorescence resonance energy transfer (FRET) sensor determination of free-apoCaM concentration in live cells. 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 Ca(V)1.3 and Ca(V)1.4 channels include a distal carboxy tail that resembles an enzyme competitive inhibitor that retunes channel affinity for apoCaM such that natural CaM variations affect the strength of Ca(2+) feedback modulation. Given the ubiquity of these channels, the connection between ambient CaM levels and Ca(2+) entry through channels is broadly significant for Ca(2+) homeostasis. Strategies such as ours promise key advances for the in situ analysis of signalling molecules resistant to in vitro reconstitution, such as Ca(2+) channels.

Switching of Ca2+-dependent inactivation of Ca(v)1.3 channels by calcium binding proteins of auditory hair cells.

Ca(V)1.3 channels comprise a vital subdivision of L-type Ca2+ channels: Ca(V)1.3 channels mediate neurotransmitter release from auditory inner hair cells (IHCs), pancreatic insulin secretion, and cardiac pacemaking. Fitting with these diverse roles, Ca(V)1.3 channels exhibit striking variability in their inactivation by intracellular Ca2+. IHCs show generally weak-to-absent Ca2+-dependent inactivation (CDI), potentially permitting audition of sustained sounds. In contrast, the strong CDI seen elsewhere likely provides critical negative feedback. Here, we explore this mysterious CDI malleability, particularly its comparative weakness in hair cells. At baseline, heterologously expressed Ca(V)1.3 channels exhibit intense CDI, wherein each lobe of calmodulin (CaM) contributes a distinct inactivation component. Because CaM-like molecules (bearing four recognizable but not necessarily functional Ca2+-binding EF hands) can perturb the Ca2+ response of molecules regulated by CaM, we asked whether such CaM-like entities could influence CDI. We find that CaM-like calcium-binding protein (CaBP) molecules are clearly expressed within the organ of Corti. In particular, the rare subtype CaBP4 is specific to IHCs, and CaBP4 proves capable of eliminating even the potent baseline CDI of Ca(V)1.3. CaBP4 thereby represents a plausible candidate for moderating CDI within IHCs.

Quantifying macromolecular interactions in living cells using FRET two-hybrid assays.

Förster resonance energy transfer (FRET) is a versatile method for analyzing protein-protein interactions within living cells. This protocol describes a nondestructive live-cell FRET assay for robust quantification of relative binding affinities for protein-protein interactions. Unlike other approaches, our method correlates the measured FRET efficiencies to relative concentration of interacting proteins to determine binding isotherms while including collisional FRET corrections. We detail how to assemble and calibrate the equipment using experimental and theoretical procedures. A step-by-step protocol is given for sample preparation, data acquisition and analysis. The method uses relatively inexpensive and widely available equipment and can be performed with minimal training. Implementation of the imaging setup requires up to 1 week, and sample preparation takes ∼1-3 d. An individual FRET experiment, including control measurements, can be completed within 4-6 h, with data analysis requiring an additional 1-3 h.

Towards a Unified Theory of Calmodulin Regulation (Calmodulation) of Voltage-Gated Calcium and Sodium Channels.

Voltage-gated Na and Ca(2+) channels represent two major ion channel families that enable myriad biological functions including the generation of action potentials and the coupling of electrical and chemical signaling in cells. Calmodulin regulation (calmodulation) of these ion channels comprises a vital feedback mechanism with distinct physiological implications. Though long-sought, a shared understanding of the channel families remained elusive for two decades as the functional manifestations and the structural underpinnings of this modulation often appeared to diverge. Here, we review recent advancements in the understanding of calmodulation of Ca(2+) and Na channels that suggest a remarkable similarity in their regulatory scheme. This interrelation between the two channel families now paves the way towards a unified mechanistic framework to understand vital calmodulin-dependent feedback and offers shared principles to approach related channelopathic diseases. An exciting era of synergistic study now looms.

Dynamic switching of calmodulin interactions underlies Ca2+ regulation of CaV1.3 channels.

Calmodulin regulation of CaV channels is a prominent Ca(2+) feedback mechanism orchestrating vital adjustments of Ca(2+) entry. The long-held structural correlation of this regulation has been Ca(2+)-bound calmodulin, complexed alone with an IQ domain on the channel carboxy terminus. Here, however, systematic alanine mutagenesis of the entire carboxyl tail of an L-type CaV1.3 channel casts doubt on this paradigm. To identify the actual molecular states underlying channel regulation, we develop a structure-function approach relating the strength of regulation to the affinity of underlying calmodulin/channel interactions, by a Langmuir relation (individually transformed Langmuir analysis). Accordingly, we uncover frank exchange of Ca(2+)-calmodulin to interfaces beyond the IQ domain, initiating substantial rearrangements of the calmodulin/channel complex. The N-lobe of Ca(2+)-calmodulin binds an N-terminal spatial Ca(2+) transforming element module on the channel amino terminus, whereas the C-lobe binds an EF-hand region upstream of the IQ domain. This system of structural plasticity furnishes a next-generation blueprint for CaV channel modulation.