STAC proteins associate to the IQ domain of Ca(V)1.2 and inhibit calcium-dependent inactivation

The adaptor proteins STAC1, STAC2, and STAC3 represent a newly identified family of regulators of voltage-gated calcium channel (Ca(V)) trafficking and function. The skeletal muscle isoform STAC3 is essential for excitation–contraction coupling and its mutation causes severe muscle disease. Recently, two distinct molecular domains in STAC3 were identified, necessary for its functional interaction with Ca(V)1.1: the C1 domain, which recruits STAC proteins to the calcium channel complex in skeletal muscle triads, and the SH3-1 domain, involved in excitation–contraction coupling. These interaction sites are conserved in the three STAC proteins. However, the molecular domain in Ca(V)1 channels interacting with the STAC C1 domain and the possible role of this interaction in neuronal Ca(V)1 channels remained unknown. Using Ca(V)1.2/2.1 chimeras expressed in dysgenic (Ca(V)1.1(−/−)) myotubes, we identified the amino acids 1,641–1,668 in the C terminus of Ca(V)1.2 as necessary for association of STAC proteins. This sequence contains the IQ domain and alanine mutagenesis revealed that the amino acids important for STAC association overlap with those making contacts with the C-lobe of calcium-calmodulin (Ca/CaM) and mediating calcium-dependent inactivation of Ca(V)1.2. Indeed, patch-clamp analysis demonstrated that coexpression of either one of the three STAC proteins with Ca(V)1.2 opposed calcium-dependent inactivation, although to different degrees, and that substitution of the Ca(V)1.2 IQ domain with that of Ca(V)2.1, which does not interact with STAC, abolished this effect. These results suggest that STAC proteins associate with the Ca(V)1.2 C terminus at the IQ domain and thus inhibit calcium-dependent feedback regulation of Ca(V)1.2 currents.