SignificanceSkeletal muscle contraction is a tightly orchestrated event that starts with the depolarization of the T-tubular membrane. At the center is a functional and mechanical coupling between two membrane proteins: L-type voltage-gated calcium channels, located in the plasma membrane, and ryanodine receptors, located in the membrane of the sarcoplasmic reticulum. How exactly these proteins associate has remained a mystery, but recent reports have highlighted a key role for the STAC3 adaptor protein in this process. Here, we provide structural snapshots of the three STAC isoforms and identify a cytosolic loop of two CaV isoforms as a functional interaction site. A mutation linked to Native American myopathy is at the interface and abolishes the interaction.

Presynaptic α 2 δ subunits of voltage-gated calcium channels regulate channel abundance and are involved in glutamatergic synapse formation. However, little is known about the specific functions of the individual α 2 δ isoforms and their role in GABAergic synapses. Using primary neuronal cultures of embryonic mice of both sexes, we here report that presynaptic overexpression of α 2 δ-2 in GABAergic synapses strongly increases clustering of postsynaptic GABA A Rs. Strikingly, presynaptic α 2 δ-2 exerts the same effect in glutamatergic synapses, leading to a mismatched localization of GABA A Rs. This mismatching is caused by an aberrant wiring of glutamatergic presynaptic boutons with GABAergic postsynaptic positions. The trans-synaptic effect of α 2 δ-2 is independent of the prototypical cell-adhesion molecules α-neurexins (α-Nrxns); however, α-Nrxns together with α 2 δ-2 can modulate postsynaptic GABA A R abundance. Finally, exclusion of the alternatively spliced exon 23 of α 2 δ-2 is essential for the trans-synaptic mechanism. The novel function of α 2 δ-2 identified here may explain how abnormal α 2 δ subunit expression can cause excitatory–inhibitory imbalance often associated with neuropsychiatric disorders. SIGNIFICANCE STATEMENT Voltage-gated calcium channels regulate important neuronal functions such as synaptic transmission. α 2 δ subunits modulate calcium channels and are emerging as regulators of brain connectivity. However, little is known about how individual α 2 δ subunits contribute to synapse specificity. Here, we show that presynaptic expression of a single α 2 δ variant can modulate synaptic connectivity and the localization of inhibitory postsynaptic receptors. Our findings provide basic insights into the development of specific synaptic connections between nerve cells and contribute to our understanding of normal nerve cell functions. Furthermore, the identified mechanism may explain how an altered expression of calcium channel subunits can result in aberrant neuronal wiring often associated with neuropsychiatric disorders such as autism or schizophrenia.

The adaptor protein STAC3 is essential for skeletal muscle excitation-contraction (EC) coupling and a mutation in the STAC3 gene has been linked to a severe muscle disease, Native American myopathy (NAM). However the function of STAC3, its interaction partner, and the mode of interaction within the EC-coupling complex remained elusive. Here we demonstrate that STAC3 forms a stable interaction with the voltage-sensor of EC-coupling, CaV1.1, and that this interaction depends on a hitherto unidentified protein-protein binding pocket in the C1 domain of STAC3. While the NAM mutation does not affect the stability of the STAC3-CaV1.1 interaction, mutation of two crucial residues in the C1 binding pocket increases the turnover of STAC3 in skeletal muscle triads. Thus, the C1 domain of STAC3 is responsible for its stable incorporation into the CaV1.1 complex, whereas the SH3 domain containing the NAM mutation site may be involved in low-affinity functional interactions in EC-coupling.