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Junctophilins (JPH) are a class of proteins found at junctions between the plasma membrane and the endoplasmic or sarcoplasmic reticulum, allowing for communications between proteins embedded in different membranes. JPHs have been proposed to interact with lipids as well as several ion channels, allowing for specialized communication between them. The JPH3 isoform is the target for repeats that cause Huntington’s disease-like 2, whereas JPH2 is a hot spot for mutations linked to cardiomyopathy. Here we present crystal structures of two JPH isoforms, which resemble a twisted skeleton with ribs formed by membrane occupation recognition nexus repeats, and a backbone built by a long α-helix. We captured the structure of a complex between JPH2 and a C-terminal binding site in the L-type calcium channel (CaV1.1) and show that this interaction is required for clustering of these channels and for robust muscle excitation–contraction coupling. Over 80 sequence variants linked to cardiomyopathy are found in different structurally important regions of JPH2, most of which affect stabilizing interactions. A subset directly affects the interaction with the L-type calcium channel. In parallel, sequence variants in the L-type calcium channel, linked to cardiac arrhythmia, also affect critical interactions. [ABSTRACT FROM AUTHOR]

Ca21-release channels are giant membrane proteins that control the release of Ca21 from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate receptors (IP 3 Rs), are evolutionarily related and are both activated by cytosolic Ca21. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca21, other major triggers include IP3 for the IP3Rs and depolarization of the plasma membrane for a particular RyR subtype expressed in skeletal muscle. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights into the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of posttranslational modifications, additional binding partners, and the higher order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca21-release channels and how this informs on their function. [ABSTRACT FROM AUTHOR]

Excitation-contraction (EC) coupling in skeletal muscle requires functional and mechanical coupling between L-type voltage-gated calcium channels (CaV1.1) and the ryanodine receptor (RyR1). Recently, STAC3 was identified as an essential protein for EC coupling and is part of a group of three proteins that can bind and modulate L-type voltage-gated calcium channels. Here, we report crystal structures of tandem-SH3 domains of different STAC isoforms up to 1.2-Å resolution. These form a rigid interaction through a conserved interdomain interface. We identify the linker connecting transmembrane repeats II and III in two different CaV isoforms as a binding site for the SH3 domains and report a crystal structure of the complex with the STAC2 isoform. The interaction site includes the location for a disease variant in STAC3 that has been linked to Native American myopathy (NAM). Introducing the mutation does not cause misfolding of the SH3 domains, but abolishes the interaction. Disruption of the interaction via mutations in the II-III loop perturbs skeletal muscle EC coupling, but preserves the ability of STAC3 to slow down inactivation of CaV1.2. [ABSTRACT FROM AUTHOR]

L-type voltage-gated calcium channels (VGCCs) are multisubunit membrane proteins that regulate calcium influx into excitable cells. Within the last two years there have been four separate reports describing the structure of the skeletal muscle VGCC determined by electron microscopy and single particle analysis methods. There are some discrepancies between the structures, as well as reports for both monomeric and dimeric forms of the channel. This article considers each of the VGCC structures in terms of similarities and differences with an emphasis upon translation of data into a biological context. [Copyright &y& Elsevier]

Toxin γ from the venom of Tityus serrulatus scorpion produces a partial block of the surface Na channel in frog muscle. This block occurs with no change in the voltage-dependence or in the kinetics of the remaining surface Na current. The partial blockade of Na channel activity occurs with no change in tubular Na currents nor in twitch tension. The maximum effect of the toxin is attained at concentrations as low as 3×10 M. Hyperpolarization to potentials more negative than the resting potential ( E=−90 mV) reduces or abolishes the effect of the toxin. Radioiodinated toxin γ binds to frog muscle membranes with a very high affinity corresponding to a dissociation constant of about 1×10 M. Data obtained with both rabbit and frog muscle indicate that toxin γ is specific for Na channels in surface membranes. Toxin γ does not seem to bind to Na channels in T-tubule membranes. The biochemical data are in good agreement with electrophysiological studies and data on contraction. There is one Tityus γ toxin binding site per tetrodotoxin binding site in surface membranes. Competition experiments have confirmed that Tityus γ toxin binds to a new toxin receptor site on the Na channel structure. This site is the same that the toxin II from Centruroides suffusus binding site, but this toxin has 100 times less affinity for the Na channel than Tityus γ toxin. [ABSTRACT FROM AUTHOR]

STAC3 is a soluble protein essential for skeletal muscle excitation-contraction (EC) coupling. Through its tandem SH3 domains, it interacts with the cytosolic II-III loop of the skeletal muscle voltage-gated calcium channel. STAC3 is the target for a mutation (W284S) that causes Native American myopathy, but multiple other sequence variants have been reported. Here, we report a crystal structure of the human STAC3 tandem SH3 domains. We analyzed the effect of five disease-associated variants, spread over both SH3 domains, on their ability to bind to the Ca V 1.1 II-III loop and on muscle EC coupling. In addition to W284S, we find the F295L and K329N variants to affect both binding and EC coupling. The ability of the K329N variant, located in the second SH3 domain, to affect the interaction highlights the importance of both SH3 domains in association with Ca V 1.1. Our results suggest that multiple STAC3 variants may cause myopathy. • Multiple variants reduce binding affinity to Ca v 1.1 and calcium transients • STAC3 tandem SH3 domains each consist of five-stranded antiparallel beta sheets • Both STAC3 SH3 domains are important for interaction with Cav1.1 • MD simulations highlight similarities between STAC2 and STAC3 binding Ca V 1.1 Combining X-ray crystallography, protein-interaction studies, EC-coupling assays, and molecular dynamics simulations, Rufenach et al. study the effects of STAC3 variants on structure and function. Several variants cause myopathy by disturbing the interaction with Ca v 1.1. They report a high-resolution structure of the human STAC3 tandem SH3 domains. [ABSTRACT FROM AUTHOR]

Ryanodine Receptors are Ca2+ release channels expressed in the Endoplasmic and Sarcoplasmic Reticulum membranes. Gong et al [1] reported cryo-EM structures of the cardiac RyR2 complexed to Calmodulin, which can downregulate channel opening. Haji-Ghassemi et al [2] report crystal structures of an RyR2 domain with PKA, a kinase promoting channel opening. [ABSTRACT FROM AUTHOR]