Alternative Splice Variant of γ-Calmodulin-Dependent Protein Kinase II Alters Activation by Calmodulin

Calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous, multifunctional enzyme family involved in the regulation of a variety of Ca2+-signaling pathways. These family members are expressed from four highly homologous genes (α, β, γ, and δ) with similar catalytic properties. Additional isoforms of each gene, created by alternative splicing of variable regions I–XI, are differentially expressed in various cell types. γB, γC, γD, γE, γF, γGs, and γH CaMKII isoforms are expressed in the biliary epithelium; however, little is known about their roles in these cells. We began our studies into the function of these variable regions by examining the effects of variable region I on kinase activation and calmodulin binding. Activities and calmodulin binding properties of γB and γGs, which differ only by the exclusion or inclusion of this region, were compared. The K0.5 for calmodulin was 2.5-fold lower for γGs than γB. In contrast, γB bound calmodulin more tightly in a calmodulin overlay assay. Mutation of variable regions Iʹs charged residue, γGs-R318E, resulted in an enzyme with intermediate activation properties but a calmodulin affinity similar to γB. Thus, variable region I appears to modulate calmodulin sensitivity, in part, through charge–charge interactions. This altered threshold of activation may modulate cellular responses to gradients of Ca2+/calmodulin in the biliary tract.