Four subunits of Ca2+/calmodulin-dependent protein kinase II (CaM KII) have several isoforms, which differ in the variable domain. We previously reported that all subunits were highly expressed in rat striatal neurons. To examine intracellular distributions of CaM KII subunits in the rat striatal neurons, we performed immunoblot analysis with antibodies specific to each subunit in cell extracts from the rat striatum after continuous sucrose density gradient fractionation. The α subunit, but not the β, γ, or δ subunits, was colocalized with synapsin I, and each subunit showed a distinct distribution pattern in the fractions. To examine further the intracellular distributions of CaM KII isoforms in the same subunit, we established NG108-15 cells stably expressing δ1, δ3, and δ4 isoforms and examined distributions of the δ and γ isoforms in these cell lines after fractionation. Each of the overexpressed exogenous δ isoforms showed a distinct distribution pattern. The endogenous δ2 was colocalized with the overexpressed δ1, δ3, and δ4 isoforms. However, the endogenous γB/γC isoforms were not colocalized with the overexpressed δ isoforms. Furthermore, the endogenous δ1 was concentrated in the microsomal fraction from the rat striatum. With the results taken together, it is suggested that CaM KII forms oligomers between isoforms in the same subunit but not in different subunits. The variable domain of CaM KII isoforms might possibly be responsible for targeting to certain intracellular compartments.
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