Alternative Splice Variants of the Human PKR Protein Kinase Possessing Different 5′-Untranslated Regions: Expression in Untreated and Interferon-Treated Cells and Translational Activity

AbstractThe double-stranded RNA-dependent protein kinase PKR is an interferon-inducible enzyme that possesses antiviral and antiproliferative activities. We examined expression of PKR transcripts in human placenta tissue and cultured human amnion U cells. Alternative exon 2 structures were identified and characterized that possess different functional activities. Cloning and sequence analyses of 5′-RACE cDNAs from human placenta established a linkage between exon 1 and three alternative exon 2 structures that constitute, together with part of exon 3, the 5′-untranslated region of the PKR mRNA. The alternative splice variants of exon 2 were designated Ex2α (83 nucleotides), Ex2β (167 nucleotides), and Ex2γ (401 nucleotides). All three exon 2 variants were present in placenta tissue. However, only the Ex2α and Ex2β forms were detectable in the amnion U cell line. Nuclease protection analysis revealed that the Ex2β form was slightly more abundant than the Ex2α form, in both placenta tissue and U cells. Interferon treatment of U cells increased the level of both Ex2α and Ex2β RNA by ∼5-fold. The translational activities, measured in a luciferase reporter assay, of RNA transcripts possessing the Ex2α and Ex2β forms of the PKR 5′-UTR were comparable to each other and more efficient than those with the Ex2γ form