Spatial adjacency of β-subunit governs F-actin binding of CaMKIIα/β heterooligomers: A statistical mechanical modeling

Abstract

The α and β isoforms of CaMKII are central orchestrators of the neuronal cytoskeleton, assembling into complex heterododecamers to regulate dendritic spine morphology. While the direct binding of CaMKIIβ to F-actin is crucial for structural plasticity, the physical principles governing this multivalent interaction remain poorly understood. Leveraging our previous high-speed atomic force microscopy (HS-AFM) observations that β subunits preferentially occupy adjacent positions within the CaMKIIα/β heterooligomer, we developed a statistical mechanics-based mathematical model to elucidate the CaMKII-actin interface. Our model successfully captures the in vivo correlation between CaMKIIα/β expression ratios and F-actin affinity, revealing that β-subunit adjacency–rather than simple stoichiometry–is the primary determinant of stable binding. We demonstrate that this spatial clustering significantly enhances binding avidity by optimizing the geometric fit to the F-actin filament, a process further modulated by the intrinsic flexibility of the β-subunit linkers. These findings identify the non-random spatial organization of CaMKII subunits as a key structural mechanism for tuning the neuronal cytoskeleton, providing a quantitative framework for understanding how protein complex architecture governs biological function.

Caption of Graphical Abstract

A statistical mechanical model was developed to estimate the number of CaMKIIα/β bound to F-actin based on free energy calculations using a shifted Lennard-Jones potential. The model was applied to CaMKIIα/β heterooligomers with adjacent versus dispersed β-subunit arrangements interacting with an F-actin model. The number of CaMKIIα/β bound to F-actin was found to be 7.3-fold higher for the adjacent arrangement than for the dispersed arrangement. These findings demonstrate that the adjacency of β subunits within the CaMKII dodecamer is essential for stable F-actin binding.