Calcium modulates the domain flexibility and function of an α-actinin similar to the ancestral α-actinin.

Abstract

The actin cytoskeleton, a dynamic network of actin filaments and associated F-actin-binding proteins, is fundamentally important in eukaryotes. α-Actinins are major F-actin bundlers that are inhibited by Ca²⁺ in nonmuscle cells. Here we report the mechanism of Ca²⁺-mediated regulation of <i>Entamoeba histolytica</i> α-actinin-2 (<i>Eh</i>Actn2) with features expected for the common ancestor of <i>Entamoeba</i> and higher eukaryotic α-actinins. Crystal structures of Ca²⁺-free and Ca²⁺-bound <i>Eh</i>Actn2 reveal a calmodulin-like domain (CaMD) uniquely inserted within the rod domain. Integrative studies reveal an exceptionally high affinity of the <i>Eh</i>Actn2 CaMD for Ca²⁺, binding of which can only be regulated in the presence of physiological concentrations of Mg²⁺ Ca²⁺ binding triggers an increase in protein multidomain rigidity, reducing conformational flexibility of F-actin-binding domains via interdomain cross-talk and consequently inhibiting F-actin bundling. In vivo studies uncover that <i>Eh</i>Actn2 plays an important role in phagocytic cup formation and might constitute a new drug target for amoebic dysentery.