The nuclear deltaB isoform of Ca2+/calmodulin-dependent protein kinase II regulates atrial natriuretic factor gene expression in ventricular myocytes

Nuclear localization of the delta subunit of Ca2+/calmodulin-dependent protein kinase II in rat cerebellar granule cells

To examine the physiological roles of the delta subunit of Ca2+/calmodulin-dependent protein kinase II (CaM kinase IIdelta) in brain, we examined the localization of CaM kinase IIdelta in the rat brain. A specific antibody to CaM kinase IIdelta1-delta4 isoforms was prepared by immunizing rabbits with a synthesized peptide corresponding to the unique carboxyl-terminal end of these isoforms. The prepared antibody did not recognize the alpha, beta, and gamma subunits, which were each overexpressed in NG108-15 cells. Immunoblot analysis on various regions and the nuclear fractions from rat brains suggested that some isoforms of CaM kinase IIdelta1-delta4 were abundant in the nucleus in the cerebellum. Total RNA from the cerebellum was analyzed by RT-PCR with a primer pair from variable domain 1 to variable domain 2. We detected the three PCR products delta3.1, delta3.4, and delta3 that contained the nuclear localization signal. These CaM kinase IIdelta3 isoforms were localized in the nuclei in transfected NG108-15 cells. Immunohistochemical study suggested the existence of these isoforms in the nuclei in cerebellar granule cells. These results suggest that CaM kinase IIdelta3 isoforms are involved in nuclear Ca2+ signaling in cerebellar granule cells.

alphaKAP is an anchoring protein for a novel CaM kinase II isoform in skeletal muscle

Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) is present in a membrane-bound form that phosphorylates synapsin I on neuronal synaptic vesicles and the ryanodine receptor at skeletal muscle sarcoplasmic reticulum (SR), but it is unclear how this soluble enzyme is targeted to membranes. We demonstrate that alphaKAP, a non-kinase protein encoded by a gene within the gene of alpha-CaM kinase II, can target the CaM kinase II holoenzyme to the SR membrane. Our results indicate that alphaKAP (i) is anchored to the membrane via its N-terminal hydrophobic domain, (ii) can co-assemble with catalytically competent CaM kinase II isoforms and target them to the membrane regardless of their state of activation, and (iii) is co-localized and associated with rat skeletal muscle CaM kinase II in vivo. alphaKAP is therefore the first demonstrated anchoring protein for CaM kinase II. CaM kinase II assembled with alphaKAP retains normal enzymatic activity and the ability to become Ca2+-independent following autophosphorylation. A new variant of beta-CaM kinase II, termed betaM-CaM kinase II, is one of the predominant CaM kinase II isoforms associated with alphaKAP in skeletal muscle SR.

Phosphorylation at the nuclear localization signal of Ca2+/calmodulin-dependent protein kinase II blocks its nuclear targeting

Translocation of protein kinases with broad substrate specificities between different subcellular compartments by activation of signaling pathways is an established mechanism to direct the activity of these enzymes toward particular substrates. Recently, we identified two isoforms of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), which are targeted to the nucleus by an alternatively spliced nuclear localization signal (NLS). Here we report that cotransfection with constitutively active mutants of CaM kinase I or CaM kinase IV specifically blocks nuclear targeting of CaM kinase II as a result of phosphorylation of a Ser immediately adjacent to the NLS of CaM kinase II. Both CaM kinase I and CaM kinase IV are able to phosphorylate this Ser residue in vitro, and mutagenesis studies suggest that this phosphorylation is both necessary and sufficient to block nuclear targeting. Furthermore, we provide experimental evidence that introduction of a negatively charged residue at this phosphorylation site reduces binding of the kinase to an NLS receptor in vitro, thus providing a mechanism that may explain the blockade of nuclear targeting that we have observed in situ.

The role of Ca2+/calmodulin-dependent protein kinases within the nucleus

Stimulation of cells by Ca(2+)-linked signaling agents increases Ca2+ levels within both the cell cytosol and nucleus. The multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) family, consisting of CaM kinases I, II and IV, have all been detected within the nucleus and each may serve as a mediator of nuclear Ca2+ signals. Certain isoforms of the large multimeric CaM kinase II are targeted to the nucleus as a result of an alternatively spliced nuclear localization signal. By contrast, CaM kinases I and IV are monomeric and likely gain nuclear access by passive diffusion through nuclear pores. These kinases have activation properties which may allow them to discriminate between Ca2+ signals which differ in their spike frequency, amplitude and duration. In addition, these kinases have the ability to control gene expression through the phosphorylation of key regulatory sites on nuclear transcription factors. CaM kinases may thus serve to decode Ca2+ signals to the nucleus in order to produce a multitude of cellular responses including control of cell cycle, apoptosis and synaptic efficacy.