Casein Kinase I α and αL:  Alternative Splicing-Generated Kinases Exhibit Different Catalytic Properties

Casein kinase I (CKI) is a family of serine/threonine protein kinases found in all eukaryotes examined to date. Here, the rat CKI isoforms α and αL were cloned and expressed in both eukaryotic and prokaryotic systems. Characterization of the genomic DNA flanking the exon unique to CKIαL demonstrated that CKIα and CKIαL arise by the alternative splicing of a common pre-mRNA molecule. To the best of our knowledge, the αL isoform is the only known active serine/threonine kinase to contain an insert within its catalytic domain. Tissue distribution of each splicing isoform was examined by RT−PCR, immunoprecipitation, and Western blotting. Both isoforms were expressed in all tissues tested but at different levels. Bacterially expressed CKIα isoforms were active and therefore biochemically characterized. CKIα and CKIαL proteins were demonstrated to have casein kinase I catalytic properties. More importantly, the recombinant isoform proteins exhibited differences in binding and activity toward common CKI substrates. These observations demonstrate that the αL insert within the kinase domain modulates substrate kinetics. These kinetic differences suggest that CKIα and CKIαL may perform different biological roles.