Calcium-calmodulin-dependent protein kinase II (CaMKII) plays a critical role in animal learning and memory (Giese et al. 1998). CaMKII operates as a multi-subunit holoenzyme and can detect the frequency of incident Ca2+ pulse trains entering the cell (Chao et al. 2011; De Koninck and Schulman 1998). CaMKII holoenzymes become destabilized and appear to exchange subunits following catalytic activation, which results in the spread of kinase activity to inactive holoenzymes (Stratton et al. 2014; Bhattacharyya et al. 2016). This phenomenon is not fully understood and is under active investigation.
This work focuses on the structural and biochemical characterization of CaMKII hub domain assemblies, which are essential to holoenzyme formation (Shen and Meyer 1998; Kolb et al. 1998). The crystal structure of a dodecameric human CaMKII-α hub domain assembly shows how the protein may be able to sense the activation state of the CaMKII kinase domain. This helps explain why kinase activation destabilizes CaMKII holoenzymes and promotes subunit exchange.
The characterization of CaMKII hub domains from evolutionarily divergent organisms is also presented. Hub domains encoded by three related green algae assemble into 16-, 18-, and 20-subunit oligomers. These are the largest known CaMKII hub domain assemblies. A crystal structure of one, the Chlamydomonas reinhardtii 18-mer, revealed multiple intra-chain hydrogen bonds not present in the human isoform.
When these hydrogen bonds were engineered into the human CaMKII hub domain by mutation the protein oligomerized into larger assemblies. A larger CaMKII holoenzyme is predicted to be more resistant to deactivation by protein phosphatases and thus may be useful in studying the consequences of aberrant CaMKII activity. These hydrogen bonds are also predicted to inhibit subunit exchange in human CaMKII by dampening structural fluctuations in the hub domain.
Work is also described on an interfacial mutation in the hub domain that weakens the integrity of the CaMKII holoenzyme. CaMKII dimers are present in solution as a result of this mutation. Catalytic activation increases the dimer population, demonstrating that the hub domain interface weakened by the mutation is further destabilized when CaMKII is active. This suggests that dimer release is a central component of activation-triggered subunit exchange between CaMKII holoenzymes.
