Stac adaptor proteins regulate trafficking and function of muscle and neuronal L-type Ca2+ channels

Excitation-contraction (EC) coupling in skeletal muscle depends upon trafficking of Ca(V)1.1, the principal subunit of the dihydropyridine receptor (DHPR) (L-type Ca2+ channel), to plasma membrane regions at which the DHPRs interact with type 1 ryanodine receptors (RyR1) in the sarcoplasmic reticulum. A distinctive feature of this trafficking is that Ca(V)1.1 expresses poorly or not at all in mammalian cells that are not of muscle origin (e.g., tsA201 cells), in which all of the other nine Ca-V isoforms have been successfully expressed. Here, we tested whether plasma membrane trafficking of Ca(V)1.1 in tsA201 cells is promoted by the adapter protein Stac3, because recent work has shown that genetic deletion of Stac3 in skeletal muscle causes the loss of EC coupling. Using fluorescently tagged constructs, we found that Stac3 and Ca(V)1.1 traffic together to the tsA201 plasma membrane, whereas Ca(V)1.1 is retained intracellularly when Stac3 is absent. Moreover, L-type Ca2+ channel function in tsA201 cells coexpressing Stac3 and CaV1.1 is quantitatively similar to that in myotubes, despite the absence of RyR1. Although Stac3 is not required for surface expression of Ca(V)1.2, the principle subunit of the cardiac/brain L-type Ca2+ channel, Stac3 does bind to Ca(V)1.2 and, as a result, greatly slows the rate of current inactivation, with Stac2 acting similarly. Overall, these results indicate that Stac3 is an essential chaperone of Ca(V)1.1 in skeletal muscle and that in the brain, Stac2 and Stac3 may significantly modulate Ca(V)1.2 function.