Multiple Ca²⁺-calmodulin-dependent protein kinase kinases from rat brain: Purification, regulation by Ca²⁺-calmodulin, and partial amino acid sequence

We have purified to near homogeneity from rat brain two Ca²⁺-calmodulin-dependent protein kinase I (CaM kinase I) activating kinases, termed here CaM kinase I kinase-α and CaM kinase I kinase-β (CaMKIKα and CaMKIKβ, respectively). Both CaMKIKα and CaMKIKβ are also capable of activating CaM kinase IV. Activation of CaM kinase I and CaM kinase IV occurs via phosphorylation of an equivalent Thr residue within the "activation loop" region of both kinases, Thr-177 and Thr-196, respectively. The activities of CaMKIKα and CaMKIKβ are themselves strongly stimulated by the presence of Ca²⁺-CaM, and both appear to be capable of Ca²⁺-CaM-dependent autophosphorylation. Automated microsequence analysis of the purified enzymes established that CaMKIKα and -β are the products of distinct genes. In addition to rat, homologous nucleic acids corresponding to these CaM kinase kinases are present in humans and the nematode, Caenorhabditis elegans. CaMKIKα and CaMKIKβ are thus representatives of a family of enzymes, which may function as key intermediaries in Ca²⁺-CaM-driven signal transduction cascades in a wide variety of eukaryotic organisms.