K+ currents expressed from the guinea pig cardiac IsK protein are enhanced by activators of protein kinase C.

We have isolated cardiac cDNA and genomic clones encoding the guinea pig IsK protein. The deduced amino acid sequence is approximately 78% identical to the rat, mouse, and human variants of this channel, and the structure of the gene encoding the protein is also similar to that in other species. For example, the gene is present only once in the haploid genome, the protein-coding sequence is present on a single uninterrupted exon, an intron exists in the 5' untranslated domain, and multiple alternative polyadenylation sites are used in processing the transcript. Expression of the guinea pig protein in Xenopus oocytes results in a slowly activating, voltage-dependent K+ current, IsK, similar to those expressed previously from the rat, mouse, and human genes. However, in sharp contrast to the rat and mouse currents, activation of protein kinase C with phorbol esters increases the amplitude of the guinea pig IsK current, analogous to its effects on the endogenous IKs current in guinea pig cardiac myocytes. Mutagenesis of the guinea pig cDNA to alter four cytoplasmic amino acid residues alters the phenotype of the current response to protein kinase C from enhancement to inhibition, mimicking that of rat and mouse IsK currents. This mutation is consistent with reports that phosphorylation of Ser-102 by protein kinase C decreases the current amplitude. These data explain previously reported differences in the regulatory properties between recombinant rat or mouse IsK channels and native guinea pig IKs channels and provide further evidence that the IsK protein forms the channels that underlie the IKs current in the heart.