Molecular structures involved in L-type calcium channel inactivation. Role of the carboxyl-terminal region encoded by exons 40-42 in alpha1C subunit in the kinetics and Ca2+ dependence of inactivation.
The pore-forming alpha1C subunit is the principal component of the voltage-sensitive L-type Ca2+ channel. It has a long cytoplasmic carboxyl-terminal tail playing a critical role in channel gating. The expression of alpha1C subunits is characterized by alternative splicing, which generates its multiple isoforms. cDNA cloning points to a diversity of human hippocampus alpha1C transcripts in the region of exons 40-43 that encode a part of the 662-amino acid carboxyl terminus. We compared electrophysiological properties of the well defined 2138-amino acid alpha1C,77 channel isoform with two splice variants, alpha1C,72 and alpha1C,86. They contain alterations in the carboxyl terminus due to alternative splicing of exons 40-42. The 2157-amino acid alpha1C,72 isoform contains an insertion of 19 amino acids at position 1575. The 2139-amino acid alpha1C,86 has 80 amino acids replaced in positions 1572-1651 of alpha1C,77 by a non-identical sequence of 81 amino acids. When expressed in Xenopus oocytes, all three splice variants retained high sensitivity toward dihydropyridine blockers but showed large differences in gating properties. Unlike alpha1C,77 and alpha1C,72, Ba2+ currents (IBa) through alpha1C,86 inactivated 8-10 times faster at +20 mV, and its inactivation rate was strongly voltage-dependent. Compared to alpha1C,77, the inactivation curves of IBa through alpha1C,86 and alpha1C,72 channels were shifted toward more negative voltages by 11 and 6 mV, respectively. Unlike alpha1C,77 and alpha1C,72, the alpha1C,86 channel lacks a Ca2+-dependent component of inactivation. Thus the segment 1572-1651 of the cytoplasmic tail of alpha1C is critical for the kinetics as well as for the Ca2+ and voltage dependence of L-type Ca2+ channel gating.