Background: The phosphoenolpyruvate:sugar phosphotransferase system (PTS) is a bacterial and mycoplasma system responsible for the uptake of some sugars, concomitant with their phosphorylation. The sugar-specific component of the system, enzyme II (EII),consists of three domains, EIIA, EIIB and EIIC. EIIA and ELLB are cytoplasmic and EIIC is an integral membrane protein that contains the sugar-binding site. Phosphoenolpyruvate (PEP) provides the source of the phosphoryl group, which is transferred via several phosphoprotein intermediates, eventually being transferred to the internalized sugar. Along the pathway, EIIA accepts a phosphoryl group from the phosphocarrier protein HPr and transfers it to EIIB. The structure of the glucose-specific EIIA (EIIAglc) from Mycoplasma capricolum reported here facilitates understanding of the nature of the interactions between this protein and its partners.
Results: The crystal structure of EIIAglc from M. capricolum has been determined at 2.5 A resolution. two neighboring EIIAglc molecules associate with one another in a front-to-back fashion, such that Glu149 of one molecule forms electrostatic interactions with the active-site histidine residues, His90 and His75, of the other. Glu149 is therefore considered to mimic the interaction that a phosphorylated histidine of a partner protein makes with EIIA. Another interaction, an ion pair between the active-site Asp94 and Lys168 of a neighboring molecule, may be analogous to the interaction between Asp94 of EIIAglc and Arg17 of HPr. Analysis of molecular packing in this crystal, and in the crystals of two other homologous proteins from Escherichia coli and Bacillus subtilis, reveals that in all cases active-site hydrophobic residues are involved in crystal contacts, but in each case a different region of the neighboring molecule is involved. The transition-state complexes of M. capricolum EIIAglc with HPr and EIIBglc have been modeled; in each case, different structural units are shown to interact with EIIAglc. Many of the interactions are hydrophobic with no sequence specificity. The only specific interaction, other than that formed by the phosphoryl group, involves ion pairs between two invariant aspartate residues of EIIAglc and arginine/lysine residues of HPr or EIIBglc.
Conclusions: The non-discriminating nature of the hydrophobic interactions that EIIAglc forms with a variety of partners may be a consequence of the requirement for interaction with a variety of proteins that show no sequence or structural similarity. Nevertheless, specificity is provided by an ion-pair interaction that is enhanced by the apolar nature of the interface.