Regulates the activity of L-type calcium channels that contain CACNA1C as pore-forming subunit. Interacts with CACNA1C. Identified in a complex with the L-type calcium channel subunits CACNA1C, CACNA2D1 and either CACNB1 or CACNB2. Detected in heart atrium and ventricle, aorta and skeletal muscle (PubMed:11738816). Detected in heart left ventricle (PubMed:21127204). Belongs to the PMP-22/EMP/MP20 family. CACNG subfamily.