Ca2؉ /calmodulin-dependent protein kinase II (CaMK-II) isozyme variability is the result of alternative usage of variable domain sequences. Isozyme expression is cell type-specific to transduce the appropriate Ca 2؉ signals. We have determined the subcellular targeting domain of ␦ E CaMK-II, an isozyme that induces neurite outgrowth, and of a structurally similar isozyme, ␥ C CaMK-II, which does not induce neurite outgrowth. ␦ E CaMK-II co-localizes with filamentous actin in the perinuclear region and in cellular extensions. In contrast, ␥ C CaMK-II is uniformly cytosolic. Constitutively active ␦ E CaMK-II induces F-actin-rich extensions, thereby supporting a functional role for its localization. C-terminal constructs, which lack central variable domain sequences, can oligomerize and localize like full-length ␦ E and ␥ C CaMK-II. Central variable domains themselves are monomeric and have no targeting capability. The C-terminal 95 residues of ␦ CaMK-II also has no targeting capability but can efficiently oligomerize. These findings define a targeting domain for ␥ and ␦ CaMK-IIs that is in between the central variable and association domains. This domain is responsible for the subcellular targeting differences between ␥ and ␦ CaMK-IIs.

Ca(2+)/calmodulin-dependent protein kinase, type II (CaMK-II) is an enzyme encoded by four genes (alpha, beta, gamma and delta) and traditionally associated with synaptic function in the adult central nervous system, but also believed to play a role during neuronal development. P19 mouse embryonic cells are a model system for neurogenesis and primarily express isozymes of delta CaMK-II. It is not yet known whether or where delta CaMK-II is expressed in P19 neurons. Using an antibody specific for the delta CaMK-II C-terminal tail, we detected a 20-fold increase in levels of delta CaMK-II along axons after 8 days of development. This coincides with increased mRNA and protein levels of delta(C) CaMK-II, which contains the alternative tail. This follows the initial stages of neurite outgrowth and beta(3) tubulin expression, which occur after 4 days. delta CaMK-II co-localizes with the axonal protein GAP-43, but not the dendritic microtubule-associated protein MAP-2, a known substrate of alpha CaMK-II. Like delta CaMK-II, GAP-43 shows increased expression after 8 days. These findings demonstrate developmental regulation of the alternative C-terminal delta CaMK-II exon and implicate endogenous delta CaMK-II in axonal development in embryonic cells.