Research Report Nuclear targeting of the CaMKII anchoring protein αKAP is regulated by alternative splicing and protein kinases Heather O'Leary a,1 , Xiaomei Sui a,1 , Pei-Ju Lin b,2 , Pompeo Volpe c , K. Ulrich Bayer a, ⁎ a Department of Pharmacology, Program in Neuroscience, University of Colorado Health Science Center, P.O. Box 6511, Aurora, CO 80045-0508, USA b Department of Neurobiology, Stanford University School of Medicine, Stanford, CA 94305, USA c Dipartmento di Scienze Biomediche Sperimentali dell'Universita di Padova and IIM, 35121 Padova, Italy ARTICLE INFO ABSTRACT Article history: Accepted 26 February 2006 Available online 17 April 2006 αKAP is an anchoring protein for the Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) and is encoded within the same gene as the CaMKIIα isoform. αKAP co-assembles with CaMKII and targets such heteromers to the membrane of the sarcoplasmic reticulum, where CaMKII can regulate Ca 2+ homeostasis. CaMKII has also nuclear functions in skeletal muscle, however, the nuclear targeting mechanism has been elusive. We show here that developmentally regulated splicing of exon Eα B generates a functional nuclear localization signal (NLS) in αKAP B , the dominant αKAP variant in mature muscle. The αKAP A variant lacks the NLS and dominates in developing muscle before and around birth. Both αKAP variants localize to membranes, but a small fraction of αKAP B is additionally found in the nucleus. Indeed, α-karyopherins that mediate nuclear import bound to αKAP B but not αKAP A in vitro. When the N-terminal membrane anchor of αKAP was deleted, localization of αKAP B but not αKAP A became predominantly nuclear. Co-expression of constitutively active CaMKI and IV, which do not bind to αKAP, interfered with nuclear localization of αKAP B . CaMKIIα was found essentially exclusively in the cytoplasm when expressed in cell lines but was targeted to the nucleus when co-expressed with the nuclear form of αKAP B . Thus, nuclear targeting of cytoplasmic CaMKII isoforms by αKAP may be regulated by developmentally controlled alternative splicing and by protein kinases. © 2006 Elsevier B.V. All rights reserved. Keywords: Targeting Kinase Muscle Nucleus Alternative splicing 1. Introduction The Ca 2+ /calmodulin-dependent protein kinase II (CaMKII), a Ser/Thr kinase with a broad substrate spectrum, is a major mediator of intracellular Ca 2+ signals (for a review, see Griffith, 2004; Hudmon and Schulman, 2002; Lisman et al., 2002; Soderling et al., 2001). CaMKII is ubiquitously expressed but is most abundant in excitable tissues such as brain, heart and skeletal muscle (Bayer et al., 1999; Tobimatsu and Fujisawa, 1989). Four different isoforms of CaMKII (α, β, γ and δ) are en- coded by distinct genes, and alternative splicing gives rise to more diversity (Bayer et al., 1996, 1998; Edman and Schulman, BRAIN RESEARCH 1086 (2006) 17 – 26 ⁎ Corresponding author. Fax: +1 303 315 7097. E-mail address: ulli.bayer@uchsc.edu (K.U. Bayer). URL: http://www2.uchsc.edu/pharm/faculty/bayer.asp (K.U. Bayer). 1 These authors contributed equally to this work and are listed in alphabetical order. 2 Present address: Stem Cell and Cell Theraphy Program, Biomedical Engineering Research Laboratories, Industrial Technology Research Institute, Chutung, Taiwan 310. 0006-8993/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2006.02.120 available at www.sciencedirect.com www.elsevier.com/locate/brainres