Biophysical analysis of the dynamics of calmodulin interactions with neurogranin and Ca /calmodulin-dependent kinase II.

Calmodulin (CaM) functions depend on interactions with CaM-binding proteins, regulated by Ca2+. Induced structural changes influence the affinity, kinetics, and specificities of the interactions. The dynamics of CaM interactions with neurogranin (Ng) and the CaM-binding region of Ca2+/calmodulin-dependent kinase II (CaMKII ) have been studied using biophysical methods. These proteins have opposite Ca2+ dependencies for CaM binding. Surface plasmon resonance biosensor analysis confirmed that Ca2+ and CaM interact very rapidly, and with moderate affinity ( KDSPR=3μM). Calmodulin-CaMKII interactions were only detected in the presence of Ca2+, exhibiting fast kinetics and nanomolar affinity ( KDSPR=7.1nM). The CaM-Ng interaction had higher affinity under Ca2+-depleted ( KDSPR=480nM,k1=3.4×105M-1s-1 and k = 1.6 × 10 s ) than Ca2+-saturated conditions ( KDSPR=19μM). The IQ motif of Ng (Ng ) had similar affinity for CaM as Ng under Ca2+-saturated conditions ( KDSPR=14μM), but no interaction was seen under Ca2+-depleted conditions. Microscale thermophoresis using fluorescently labeled CaM confirmed the surface plasmon resonance results qualitatively, but estimated lower affinities for the Ng ( KDMST=890nM) and CaMKII ( KDMST=190nM) interactions. Although CaMKII showed expected interaction characteristics, they may be different for full-length CaMKII. The data for full-length Ng, but not Ng , agree with the current model on Ng regulation of Ca2+/CaM signaling.