Magnesium Cations Modulate Actin Bundles Formed by α ‐Actinin

ABSTRACT α ‐Actinin is an actin‐crosslinking protein that influences cytoskeletal organization by forming actin bundles or networks. Divalent cations, such as magnesium cations (Mg 2+ ), promote bundling through counterion condensation. While α ‐actinin and Mg 2+ individually promote actin bundle assembly, their combined effects on actin bundling remain unclear. Here, we determine how Mg 2+ affects α ‐actinin's binding to filaments and its subsequent bundling activities through biophysical analysis. We show that increasing Mg 2+ concentrations enhance the binding of α ‐actinin to actin filaments, producing more compact bundles instead of loose networks. Molecular dynamics simulation indicates that Mg 2+ modulates the interaction between filaments and the calponin homology 1 domain of α ‐actinin. We propose a mechanism by which divalent cations and α ‐actinin crosslinking act simultaneously to drive actin bundle formation.