THE 1-CENTER-DOT-6 ANGSTROM STRUCTURE OF HISTIDINE-CONTAINING PHOSPHOTRANSFER PROTEIN HPR FROM STREPTOCOCCUS-FAECALIS

The histidine-containing phosphocarrier protein (HPr) is a central component of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) that transports carbohydrates across the cell membrane of bacteria. The three-dimensional structure of Gram-positive Streptococcus faecalis HPr has been determined using the method of multiple isomorphous replacement. The R factor for all data is 0·156 for S. faecalis HPr at 1·6 Å resolution with very good geometry. The overall folding topology of HPr is a classical open-faced β-sandwich, consisting of four antiparallel β-strands and three α-helices. Remarkable disallowed Ramachandran torsion angles of Ala16 at the active center, revealed by the X-ray structure of S. faecalis HPr, demonstrate a unique example of torsion-angle strain that is likely involved directly in protein function. A brief report concerning the torsion-angle strain has been presented recently. A newly-determined pH 7·0 structure is shown to have the same open conformation of the active center and the same torsion-angle strain at Alal6, suggesting that pH is not responsible for the structural observations. The current structure suggests a role for residues 12 and 51 in HPr's function, since they are involved in the active center through direct and indirect hydrogen-bonding interactions with the imidazole ring of Hisl5. It is found that Ser46, the regulatory site in HPr from Gram-positive bacteria, N-caps the minor a-13 helix and is also involved in the Asn43-Ser46 β-turn. This finding, in conjunction with the proposed routes of communication between the regulatory site Ser46 and the active center in S. faecalis HPr, provides new insight into the understanding of hoxv Ser46 might function. The putative involvement of the C-terminal α-carboxyl group and the related GIy67-Glu70 reverse β-turn with respect to the function of HPr are described. © 1994.