Functional and structural diversity of variable domain of Ca^<2+>/calmodulin-dependent protein kinase II

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Ca^<2+>/calmodulin-dependent protein kinase II (CaM kinase II) is widely distributed and may be involved in a variety of Ca^<2+>-mediated cellular processes. Four subunits, termed α, β, γ, δ, are encoded by distinct genes in eukaryotes. All subunits have high homology in the N-terminal catalytic domain and in the regulatory domain comprising calmodulin-binding and autoinhibitory sites. Various isoforms of these subunits exist as different splicing variants. The isoforms differ mainly at the end of the regulatory domain (variable domain). In this study, we identified isoforms in brain and various cultured cells with immunochemical and molecular biological techniques. In addition, their physiological roles in these cells were examined. 1. We prepared a specific antibody to δ subunit and found that δ3 was abundant in the nucleus in cerebellar granule cells. We transiently overexpressed the nuclear isoforms, αB and δ3, in NG108-15 cells and found that these isoforms were involved in the expression of brain-derived neurotrophic factor. 2. We identified all isoforms in cultured astrocytes and NG108-15 cells. Among the isoforms identified, the most abundant isoform was δ2. Immunostaining suggested that δ2 was localized predominantly at the Golgi apparatus in both cells. 3. When we examined the isoforms in mouse insulinoma MIN6 cells, the abundant isoforms were β'e and δ2 at both mRNA and protein levels. We found that δ2 and synapsin I colocalized with insulin secretory granules. We confirmed that overexpression of β'e and δ2 enhanced the phosphorylation of synapsin I and insulin secretion. These results suggest that each isoform is localized at the specific site in the cells and plays a critical role in the coupling of Ca^<2+> signals to specific cellular responces.