CaMKII binds both substrates and effectors at the active site
ABSTRACTCa2+/calmodulin dependent protein kinase II (CaMKII) is a signaling protein that is required for long-term memory formation. Ca2+/CaM activates CaMKII by binding to its regulatory segment, thereby freeing the substrate binding pocket. One exceptional feature of this kinase is that interaction with specific binding partners 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. We solved X-ray crystal structures of the CaMKII kinase domain bound to four different effectors that modulate CaMKII activity. We show that all four 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. The structures presented here shed much-needed light on CaMKII interactions. These observations will be crucial in guiding further biological experiments.