K-PIG CARDIAC I-SK PROTEIN ARE ENHANCED BY ACTIVATORS OF PROTEIN-KINASE-C( CURRENTS EXPRESSED FROM THE GUINEA)

We have isolated cardiac cDNA and genomic clones encoding the guinea p ig I-sK protein. The deduced amino acid sequence is approximate to 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 h aploid genome, the protein-coding sequence is present on a single unin terrupted exon, an intron exists in the 5' untranslated domain, and mu ltiple alternative polyadenylylation sites are used in processing the transcript. Expression of the guinea pig protein in Xenopus oocytes re sults in a slowly activating, voltage-dependent K+ current, I-sK, Simi lar 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 th e guinea pig I-sK current, analogous to its effects on the endogenous I-Ks current in guinea pig cardiac myocytes. Mutagenesis of the guinea pig cDNA to alter four cytoplasmic amino acid residues alters the phe notype of the current response to protein kinase C from enhancement to inhibition, mimicking that of rat and mouse GK currents. This mutatio n is consistent with reports that phosphorylation of Ser-102 by protei n kinase C decreases the current amplitude. These data explain previou sly reported differences in the regulatory properties between recombin ant rat or mouse I-sK channels and native guinea pig I-Ks channels and provide further evidence that the I-sK protein forms the channels tha t underlie the I-Ks current in the heart.