Molecular cloning of a tissue-specific protein kinase (Cγ) from human testis - Representing a third isoform for the catalytic subunit of cAMP-dependent protein kinase

Abstract

Two different mammalian genes for the catalytic subunit (C) of cAMP-dependent protein kinase have previously been characterized (Cα, Cβ). In the present study, we report the molecular cloning of a third isoform of C, from a human testis cDNA library, as well as the isolation of human cDNAs for Cα and Cβ. This third form of C, which we will designate Cγ, is clearly derived from a distinct gene and shows a tissue-specific expression. A close evolutionary relation between Cγ and Cα, was suggested by nucleotide homologies (86% inside the open reading frame, 81% in the 3′-untranslated region). Thus, the Cγ cDNA cross-hybridized with the 2.8 kilobase (kb) Cα mRNA, present at high levels in most human tissues, as well as with a 1.8 kb Cγ-specific mRNA, which was only found at detectable levels in human testis. However, at the amino acid level, Cα and Cβ showed a close relationship (93% homology), whereas Cγ diverged significantly from both Cα (83%) and Cβ (79%). Taken together with the tissue-specific expression of Cγ, this suggests a pressure on Cγ during evolution, acting to modulate it in a functionally specific way. Certain amino acid substitutions make Cγ a distinct member of the cAMP-dependent subfamily of protein kinases, and suggest that Cγ may be distinct in its protein substrate specificity or its interaction with the different regulatory subunits.