Catalytic subunits of Aplysia neuronal cAMP-dependent protein kinase with two different N termini

Previously, two forms of cAMP-dependent protein kinase catalytic subunit generated by mutually exclusive use of two internal exon cassettes (A1 and A2) were demonstrated in Aplysia neurons. Here, it is shown that there also exist catalytic subunits with alternative N termini derived from two exons, N1 and N2, expressed in combination with either of the internal cassettes. Processed transcripts including N1 or N2 sequences are of about equal abundance in the nervous system, arise through alternative promoter use, and encode catalytically active polypeptides. The N2 amino acid sequence is 21 residues longer than the N1 sequence and is homologous to the nonmyristoylated N terminus of the TPK1 gene product, a yeast catalytic subunit homolog. These data support the view that cAMP-dependent protein kinase activity in Aplysia neurons is produced by a complex array of regulatory and catalytic subunits that generate multiple holoenzymes with a spectrum of properties.