The CaMKII D135N mutation blocks kinase activity and reduces GluN2B binding

Three recent studies claimed that induction of long-term potentiation of synaptic strength requires structural rather than enzymatic functions of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). One study used the CaMKII D135N mutation, which was claimed to abolish enzymatic activity without affecting the structural function, that is, binding to GluN2B. We found here that the D135N mutant indeed abolished enzymatic kinase activity and autophosphorylation at T286. However, D135N mutation additionally reduced binding to GluN2B and prevented persistence of co-condensation with GluN2B. Similar to the T286A mutant, GluN2B binding of the D135N mutant could be partially rescued by AS283, an inhibitor that directly enhances GluN2B binding. This partial effect on GluN2B binding has to be taken into account when using D135N to distinguish between enzymatic versus structural functions of CaMKII. Nonetheless, as discussed here, the D135N mutant indeed supports a structural rather than enzymatic CaMKII function in long-term potentiation induction. SIGNIFICANCE STATEMENT: This study characterized the Ca²⁺/calmodulin-dependent protein kinase II D135N mutant and found abolished kinase activity and reduced GluN2B binding. These findings support the nonenzymatic role of Ca²⁺/calmodulin-dependent protein kinase II in long-term potentiation induction but provide important information for the interpretation of results obtained with this tool mutant.