Structure of the Bacillus anthracis Sortase A Enzyme Bound to Its Sorting Signal: a Flexible Amino-terminal Appendage Modulates Substrate Access

Abstract The endospore forming bacterium Bacillus anthracis causes lethal anthrax disease in humans and animals. The ability of this pathogen to replicate within macrophages is dependent upon the display of bacterial surface proteins attached to the cell wall by the Bacillus anthracis Sortase A (BaSrtA) enzyme. Previously, we discovered that the class A BaSrtA sortase contains a unique N-terminal appendage that wraps around the body of the protein to contact the enzyme's active site. To gain insight into its function, we determined the NMR structure of BaSrtA bound to a LPXTG sorting signal analog. The structure, combined with dynamics, kinetics and whole-cell protein display data suggest that the N-terminus modulates substrate access to the enzyme. We propose that it may increase the efficiency of protein display by reducing the unproductive hydrolytic cleavage of enzyme-protein covalent intermediates that form during the cell wall anchoring reaction. Notably, a key active site loop (β7/β8 loop) undergoes a disordered-to-ordered transition upon binding the sorting signal, potentially facilitating recognition of lipid II.