A novel long N-terminal isoform of human L-type Ca²⁺ channel is up-regulated by protein kinase C

Human L-type voltage-dependent Ca²⁺ channels (α_1C, or Ca_v1.2) are up-regulated by protein kinase C (PKC) in native tissues, but in heterologous systems this modulation is absent. In rat and rabbit, α_1C has two N-terminal (NT) isoforms, long and short, with variable initial segments of 46 and 16 amino acids, respectively. The initial 46 amino acids of the long-NT α_1C are crucial for PKC regulation. However, only a short-NT human α_1C is known. We assumed that a long-NT isoform of human α_1C may exist. By homology screening of human genomic DNA, we identified a stretch (termed exon 1a) highly homologous to rabbit long-NT, separated from the next known exon of α_1C (exon 1b, which encodes the alternative, short-NT) by an ∼80 kb-long intron. The predicted 46-amino acid protein sequence is highly homologous to rabbit long-NT. Reverse transcriptase PCR showed the presence of exon 1a transcript in human cardiac RNA. Expression of human long-NT α_1C in Xenopus oocytes produced Ca²⁺ channel enhanced by a PKC activator, whereas the short-NT α_1C was inhibited. The long-NT isoform may be the Ca²⁺ channel enhanced by PKC-activating transmitters in human tissues.