TRANSLATION INITIATION OF A CARDIAC VOLTAGE-GATED POTASSIUM CHANNEL BY INTERNAL RIBOSOME ENTRY

The mammalian Kv1.4 voltage-gated potassium channel mRNA contains an u nusually long (1.2 kilobases) 5'-untranslated region (UTR) and include s 18 AUG codons upstream of the authentic site of translation initiati on. Computer-predicted secondary structures of this region reveal comp lex stem-loop structures that would serve as barriers to 5' --> 3' rib osomal scanning. These features suggested that translation initiation in Kv1.4 might occur by the mechanism of internal ribosome entry, a mo de of initiation employed by a variety of RNA viruses but only a limit ed number of vertebrate genes. To test this possibility we introduced the 5'-UTR of mouse Kv1.4 mRNA into the intercistronic region of a bic istronic vector containing two tandem reporter genes, chloramphenicol acetyltransferase and luciferase, The control construct translated onl y the upstream chloramphenicol cistron in transiently transfected mamm alian cells. In contrast, the construct containing the mKv1.4 UTR effi ciently translated the luciferase cistron as well, demonstrating the p resence of an internal ribosome entry segment. Progressive 5' --> 3' d eletions localized the activity to a 3'-proximal 200-nucleotide fragme nt. Suppression of cap-dependent translation by extracts from poliovir us-infected HeLa cells in an in vitro translation assay eliminated tra nslation of the upstream cistron while allowing translation of the dow nstream cistron, Our results indicate that the 8'-untranslated region of mKv1.4 contains a functional internal ribosome entry segment that m ay contribute to unusual and physiologically important modes of transl ation regulation for this and other potassium channel genes.