STRUCTURAL EVIDENCE FOR THE EVOLUTIONARY DIVERGENCE OF MYCOPLASMA FROM GRAM-POSITIVE BACTERIA - THE HISTIDINE-CONTAINING PHOSPHOCARRIER PROTEIN

Background: The three-dimensional structures of histidine-containing p hosphocarrier protein (HPr), a member of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), have been determined from Gram-negat ive and Gram-positive bacteria. The structure of HPr reported here for Mycoplasma capricolum is the first protein structure to be determined for this class of organism. Comparative structural studies with the b acterial proteins highlight sequence-structure correlations relevant t o proposals about the evolutionary origin of mycoplasmas. Results: The crystal structure of HPr from M. capricolum has been determined and r efined at 1.8 Angstrom resolution, revealing the same overall fold as that of other HPrs of known structure. However, mycoplasma HPr resembl es HPrs from Gram-positive bacteria more closely than those from Gram- negative bacteria. As in HPrs from Bacillus subtilis and Escherichia c oli, the phosphoryl group carrier (His15) forms the N-terminal cap of a helix, but in contrast to the other crystal structures, the side cha in of the adjacent Arg17 is conformationally disordered. A sulfate ion interacts with Ser46, a residue known to be phosphorylated in a regul atory manner. Conclusions: The greater degree of structural similarity of the M. capricolum HPr to HPrs from Gram-positive rather than Gram- negative bacteria is consistent with the proposal that mycoplasma evol ved from Gram-positive bacteria. The proposal that no major conformati onal transition is required for phosphorylation oi the active-site his tidine is reinforced by comparing the crystal structures with and with out an anion in the active site. The conformational disorder of the Ar g17 side chain suggests that its guanidinium group does not have to fo rm specific interactions with other protein groups before phosphorylat ion at His15. The association of a sulfate ion with Ser46 serves as a model for HPr(Ser46 similar to P). As there is no evidence of a confor mational change accompanying Ser46 phosphorylation, the inhibitory eff ect of this event may be attributable to altered surface electrostatic s.