Listeria monocytogenes ActA protein interacts with phosphatidylinositol 4,5-bisphosphate in vitro

The N-terminal region of the Listeria monocytogenes Acta protein, in conjunction with host cell factors, is sufficient for actin polymerization at the bacterial surface. Previous data suggested that Acta could protect barbed ends from capping proteins. We tested this hypothesis by actin polymerization experiments in the presence of the Acta N-terminal fragment and capping protein. Acta does not protect barbed ends from capping protein. In contrast, this polypeptide prevents PIP₂ from inhibiting the capping activity of capping protein. Gel filtration and tryptophan fluorescence experiments showed that the purified Acta N-terminal fragment binds to PIP₂ and PIP, defining phosphoinositides as novels ligands for this functional domain of ActA. Phosphoinositide binding to the N-terminal region of Acta may induce conformational changes in Acta and/or facilitate binding of other cell components, important for ActA-induced actin polymerization.