Fig. 8: CaMKII hub assembly model.

The assembly of polymorphic hub structures from the tetramer precursor structure entails a balance between polymerization and isomerization. The polymerization of progressively larger open structures depends on dimer concentration (C), analogous to the concentration dependence of the open, spiral metazoan hub. The free energy (ΔE) for hub formation depends on the trade-off between the strain in the closed hub and the solvation of the four LCs upon dimer addition. The axes for both (C) and (ΔE) are relative. The open hub resolved in this study would require concentration-dependent reinsertion followed by ring closure to obtain the 14-mer hub, or it may close to form the 12-mer hub. The 12-mer and 14-mer hubs are the dominant forms seen in all EM studies thus far. The deviation of these structures from the intrinsic curvature found in the tetramer has the opposite sign but equal amplitude. For replica and negative-stain samples where particle concentrations are typically lower than for cryo-EM, the 12-mer hub might be the favored form. Freeze-etch replicas have reported a β 10-mer⁵⁹, but the stoichiometry was estimated from the peripheral KDs that may have been lost by proteolysis of the long β KD-AD linker. A β 16-mer has been crystallized, but EM suggests that it is a minor species (<0.4%) relative to the 12-mer (92.7%) at the concentration used in a negative stain study¹⁹. The 16-mer and the 10-mer (depicted as ? to note its existence is questionable), will be more strained than the dominant polymorphs due to the greater deviation of their curvature from the intrinsic LC curvature. An even greater penalty might prohibit the formation of smaller hubs, and none have been reported.