αKAP 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 alpha KAP, 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 alpha KAP (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 colocalized and associated with rat skeletal muscle CaM kinase II in vivo, alpha KAP is therefore the first demonstrated anchoring protein for CaM kinase II, CaM kinase II assembled with alpha KAP retains normal enzymatic activity and the ability to become Ca2+-independent following autophosphorylation. A new variant of beta-CaM kinase II, termed beta(M)-CaM kinase II, is one of the predominant CaM kinase II isoforms associated with alpha KAP in skeletal muscle SR.