Differential effects of δ_B and δ_C subtypes of Ca²⁺/calmodulin-dependent protein kinase (CaMKII) on cardiomyocyte Ca²⁺ handling and survival have been suggested to result from their respective nuclear versus cytosolic localizations. CaMKIIδ subtype localization and its relationship to enzyme activation and target phosphorylation have not, however, been systematically evaluated.

To determine whether CaMKIIδ subtypes are restricted to a particular subcellular location and assess the relationship of localization to enzyme activation and function.

CaMKIIδ is highly expressed in mouse heart and cardiomyocytes and concentrated in sarcoplasmic reticulum (SR)/membrane and nuclear fractions. CaMKIIδ_B and δ_C subtypes differ by a nuclear localization sequence, but both are present in nuclear and SR/membrane fractions. Nonselective subtype distribution is also seen in mice overexpressing CaMKIIδ_B or δ_C, even in a CaMKIIδ null background. Fluorescently tagged CaMKIIδ_B expressed in cardiomyocytes concentrates in nuclei whereas δ_C concentrates in cytosol, but neither localization is exclusive. Mouse hearts exposed to phenylephrine show selective CaMKIIδ activation in the nuclear (versus SR) compartment, whereas caffeine selectively activates CaMKIIδ in SR (versus nuclei), independent of subtype. Compartmentalized activation extends to functional differences in target phosphorylation at CaMKII sites: phenylephrine increases histone deacetylase 5 phosphorylation (Ser498) but not phospholamban (Thr17), whereas the converse holds for caffeine.

These studies demonstrate that CaMKIIδ_B and δ_C are not exclusively restricted to the nucleus and cytosol and that spatial and functional specificity in CaMKIIδ activation is elicited by mobilization of different Ca²⁺ stores rather than by compartmentalized subtype localization.