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 ofEntamoeba histolyticaα-actinin-2 (EhActn2) with features expected for the common ancestor ofEntamoebaand higher eukaryotic α-actinins. Crystal structures of Ca2+-free and Ca2+-boundEhActn2 reveal a calmodulin-like domain (CaMD) uniquely inserted within the rod domain. Integrative studies reveal an exceptionally high affinity of theEhActn2 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 thatEhActn2 plays an important role in phagocytic cup formation and might constitute a new drug target for amoebic dysentery.