CASEIN-KINASE-I-ALPHA AND CASEIN-KINASE-ALPHA-L - 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 a lpha and alpha L were cloned and expressed in both eukaryotic and prok aryotic systems. Characterization of the genomic DNA flanking the exon unique to CKI alpha L demonstrated that CKI alpha and CKI alpha L ari se by the alternative splicing of a common pre-mRNA molecule. To the b est of our knowledge, the alpha L isoform is the only known active ser ine/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 expresse d in all tissues tested but at different levels. Bacterially expressed CKI alpha isoforms were active and therefore biochemically characteri zed. CKI alpha and CKI alpha L proteins were demonstrated to have case in kinase I catalytic properties. More importantly, the recombinant is oform proteins exhibited differences in binding and activity toward co mmon CKI substrates. These observations demonstrate that the alpha L i nsert within the kinase domain modulates substrate kinetics. These kin etic differences suggest that CKI alpha and CKI alpha L may perform di fferent biological roles.