CaMKII generates actin bundle morphology and mechanics distinct from canonical bivalent cross-linkers

Actin cross-linkers play a vital role in shaping and modifying the actin cytoskeleton, allowing for structural variety and adaptable remodeling. Among these cross-linkers, calcium/calmodulin-dependent protein kinase II (CaMKII) displays a unique duality as both a kinase and a structural binder of actin within dendritic spines. This is crucial for maintaining the structural integrity of these spines via its beta subunit.

To delve into the structural role of CaMKII, the researchers quantified the morphology and mechanical properties of actin bundles formed by CaMKII, and directly compared them against bundles formed by canonical bivalent cross-linkers: actinin and fascin. Utilizing fluorescence microscopy, they assessed contour length, straightness ratio, and persistence length across varying concentrations of each cross-linker.

The findings showed that fascin produced bundles that became progressively shorter, straighter, and stiffer with increasing concentration. Conversely, actinin bundles remained relatively unchanged regardless of concentration. On the other hand, as CaMKII concentration increased, both bundle straightness and persistence length decreased, indicating a unique trend of enhanced flexibility at higher levels of CaMKII.

To further investigate the structural foundations behind these observed mechanical behaviors, the researchers complemented their experimental data with coarse-grained simulations of actin bundles cross-linked by these proteins. The simulations revealed that CaMKII-generated bundles retained a more substantial amount of curvature in their average configuration compared to those formed by actinin or fascin.

This suggests that the mechanics of these bundles are significantly influenced by factors beyond mere thermal bending fluctuations.

The behavior of CaMKII bundles exhibited characteristics consistent with additional structural complexity originating from their multivalent architecture and flexible linker domains. In summary, the experimental and computational findings collectively highlight CaMKII as a structurally and mechanically distinct actin cross-linker.

The team proposes that the unique combination of multivalency and linker flexibility in CaMKII enables it to function as a molecular structural pivot. This facilitates the production of adaptable actin assemblies that are mechanically compatible with the dynamic remodeling required for dendritic spine plasticity.