A Phosphatidylserine-binding Site in the Cytosolic Fragment of Clostridium sordellii Lethal Toxin Facilitates Glucosylation of Membrane-bound Rac and Is Required for Cytotoxicity*

Large clostridial toxins glucosylate some small G proteins on a threonine residue, thereby preventing their interactions with effector molecules and regulators. We show that the glucosyltransferase domain of lethal toxin from Clostridium sordellii (LTcyt; amino acids 1–546), which is released into the cytosol during cell infection, binds preferentially to liposomes containing phosphatidylserine as compared with other anionic lipids. The binding of LTcyt to phosphatidylserine increases by two orders of magnitude the rate of glucosylation of liposome-bound geranyl-geranylated Rac-GDP. Limited proteolysis and deletion studies show that the binding site for phosphatidylserine lies within the first 18 N-terminal residues of LTcyt. Deletion of these residues abolishes the effect of phosphatidylserine on the activity of LTcyt on liposome-bound geranyl-geranylated Rac-GDP and prevents the morphological effects induced by LTcyt microinjection into various cells, but it does not affect the intrinsic activity of LTcyt on non-geranyl-geranylated Rac-GDP in solution. We conclude that the avidity of LTcyt for phosphatidylserine facilitates its targeting to the cytosolic leaflet of cell membranes and, notably, the plasma membrane, where this anionic lipid is abundant and where several targets of lethal toxin reside.