Proteolytic processing of the C terminus of the α(1C) subunit of L-type calcium channels and the role of a proline-rich domain in membrane tethering of proteolytic fragments

Although most L-type calcium channel α(1C) subunits isolated from heart or brain are ~190-kDa proteins that lack ~50 kDa of the C terminus, the C- terminal domain is present in intact cells. To test the hypothesis that the C terminus is processed but remains functionally associated with the channels, expressed, full-length α(1C) subunits were cleaved in vitro by chymotrypsin to generate a 190-kDa C-terminal truncated protein and C-terminal fragments of 30-56 kDa. These hydrophilic C-terminal fragments remained membrane- associated. A C-terminal proline-rich domain (PRD) was identified as the mediator of membrane association. The α(1C) PRD bound to SH3 domains in Src, Lyn, Hck, and the channel β₂ subunit. Mutant α(1C) subunits lacking either ~50 kDa of the C terminus or the PRD produced increased barium currents through the channels, demonstrating that these domains participate in the previously described (Wei, X., Neely, a., Lacerda, A. E. Olcese, r., Stefani, E., Perez-Reyes, E., and Birnbaumer, L. (1994) J. Biol. Chem. 269, 1635-1640) inhibition of channel function by the C terminus.