A PROMISCUOUS BINDING SURFACE - CRYSTAL-STRUCTURE OF THE IIA DOMAIN OF THE GLUCOSE-SPECIFIC PERMEASE FROM MYCOPLASMA-CAPRICOLUM

Background: The phosphoenolpyruvate: sugar phosphotransferase system ( PTS) is a bacterial and mycoplasma system responsible for the uptake o f some sugars, concomitant with their phosphorylation. The sugar-speci fic component of the system, enzyme II (EII), consists of three domain s, EIIA, EIIB and EIIC. EIIA and EIIB are cytoplasmic and EIIC is an i ntegral membrane protein that contains the sugar-binding site. Phospho enolpyruvate (PEP) provides the source of the phosphoryl group, which is transferred via several phosphoprotein intermediates, eventually be ing transferred to the internalized sugar. Along the pathway, EIIA acc epts a phosphoryl group from the phosphocarrier protein HPr and transf ers it to EIIB. The structure of the glucose-specific EIIA (EIIA(glc)) from Mycoplasma capricolum reported here facilitates understanding of the nature of the interactions between this protein and its partners. Results: The crystal structure of EIIA(glc) from M. capricolum has be en determined at 2.5 Angstrom resolution. Two neighboring EIIA(glc) mo lecules associate with one another in a front-to-back fashion, such th at Glu149 of one molecule forms electrostatic interactions with the ac tive-site histidine residues, His90 and His75, of the other. Glu149 is therefore considered to mimic the interaction that a phosphorylated h istidine of a partner protein makes with EIIA. Another interaction, an ion pair between the active-site Asp94 and Lys168 of a neighboring mo lecule, may be analogous to the interaction between Asp94 of EIIA(glc) 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 hydropho bic residues are involved in crystal contacts, but in each case a diff erent region of the neighboring molecule is involved. The transition-s tate complexes of M. capricolum EIIA(glc) with HPr and EIIBglc have be en modeled; in each case, different structural units are shown to inte ract with EIIA(glc). Many of the interactions are hydrophobic with no sequence specificity. The only specific interaction, other than that f ormed by the phosphoryl group, involves ion pairs between two invarian t aspartate residues of EIIA(glc) and arginine/lysine residues of HPr or EIIBglc. Conclusions: The non-discriminating nature of the hydropho bic interactions that EIIA(glc) forms with a variety of partners may b e a consequence of the requirement for interaction with a variety of p roteins 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.