Cloning, characterization and expression of two alternatively splicing isoforms of Ca2+/calmodulin‐dependent protein kinase Iγ in the rat brain

Ca2+/calmodulin-dependent protein kinase I (CaMKI), originally identified as a protein kinase phoshorylating synapsin I, has been shown to constitute a family of closely related isoforms (α, β and γ). Here, we have isolated and determined the complete primary structures of two alternatively splicing isoforms of CaMKI termed CaMKIγ1 and -γ2. CaMKIγ1 and -γ2 contain an identical N-terminal catalytic domain with different C-terminal regions due to the deletion of the 425-bp nucleotide sequence of CaMKIγ1 in CaMKIγ2. In vitro kinase assay has demonstrated the marked enhancement of the Ca2+/CaM-dependent activity of CaMKIγ1 by the preincubation with Ca2+/calmodulin-dependent protein kinase kinase (CaMKK), but no significant activation of CaMKIγ2. Northern blot analysis has demonstrated the predominant expression of CaMKIγ in the brain. RT-PCR analysis has revealed similar expression patterns between CaMKIγ1 and CaMKIγ2 in various brain regions. In situ hybridization analysis has demonstrated that CaMKIγ mRNA is expressed in a distinct pattern from other isoforms of CaMKI with predominant expression in some restricted brain regions such as the olfactory bulb, hippocampal pyramidal cell layer of CA3, central amygdaloid nuclei, ventromedial hypothalamic nucleus and pineal gland. In the primary hippocampal neurons and NG108-15 cells, transfected CaMKIγ1 and -γ2 are localized primarily in the cytoplasm and neurites but not in the nucleus. These findings suggest that both isoforms of CaMKIγ may be involved in Ca2+ signal transduction in the cytoplasmic compartment of certain neuronal population.