Flexible linkers in CaMKII control the balance between activating and inhibitory autophosphorylation

The activity of Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) depends on the balance between activating and inhibitory autophosphorylation (Thr 286 and Thr 305/306, respectively, in the human α isoform). Variation in the lengths of the flexible linkers that connect the kinase domains of CaMKII to a central oligomeric hub could alter transphosphorylation rates within a holoenzyme, thereby affecting the balance of autophosphorylation outcomes. Using a single-molecule assay for visualization of CaMKII phosphorylation on glass, we show that the balance of autophosphorylation is flipped between CaMKII-α and CaMKII-β, the two principal isoforms in the brain. CaMKII-α, with a ∼30 residue kinase-hub linker, readily acquires activating autophosphorylation, which we show is resistant to removal by phosphatases. CaMKII-β, with a ∼200 residue kinase-hub linker, is biased towards inhibitory autophosphorylation. Thus, the responsiveness of CaMKII to calcium signals can be tuned by varying the relative levels of the α and β isoforms.