Ca2+/calmodulin dependent protein kinase II (CaMKII) is a signaling protein that is required for successful long-term memory formation. Ca2+/CaM activates CaMKII by binding to its regulatory segment, thereby making the substrate binding pocket available. One exceptional feature of this kinase is that two binding partners have been shown persistently activate CaMKII after the Ca2+ stimulus dissipates. The molecular details of this phenomenon are unclear. Despite having a large variety of interaction partners, the specificity of CaMKII has not been structurally well-characterized. To this end, we solved X-ray crystal structures of the CaMKII kinase domain bound to four different binding partners: a peptide substrate, a substrate/activator, a substrate/binding partner, and an inhibitor (AMPA-type glutamate receptor, NMDA-type glutamate receptor, Tiam1, and Densin-180). We show that all four binding partners use similar interactions to bind across the substrate binding pocket of the CaMKII active site. We generated a sequence alignment based on our structural observations, which revealed conserved interactions across these binding partners. The structures presented here shed much-needed light on the interaction between CaMKII and its binding partners. These observations will be crucial in guiding further biological experiments.
