FIGURE 2.

NMR solution structure of the SrtA_ΔN59-LPAT* complex. a, stereo image showing the ensemble of 40 lowest energy structures of the SrtA_ΔN59-LPAT* complex. The protein backbone heavy atoms (blue) and the covalently linked peptide (red) are shown. A yellow sphere represents the calcium ion. b, ribbon drawing of the structure of the SrtA_ΔN59-LPAT* complex. The covalently bound peptide is shown in a red ball-and-stick representation with its amino acids labeled. c, expanded view of the SrtA_ΔN59-substrate interface showing how the leucine and proline residues of the sorting signal are recognized. A ribbon drawing of the SrtA protein (white) with select side chains (yellow) is shown. d, expanded view of the positioning of the threonine and alanine residues of the sorting signal. The side chains of His¹²⁰ and Arg¹⁹⁷ are also shown, demonstrating their roles in catalysis. The panel is colored as in c. The -CH₂-SH group of the peptide is covalently bonded with the side chain thiol group of Cys¹⁸⁴ through a disulfide bridge (pale yellow spheres).