THE STRUCTURE OF AN ENERGY-COUPLING PROTEIN FROM BACTERIA, IIBCELLOBIOSE, REVEALS SIMILARITY TO EUKARYOTIC PROTEIN-TYROSINE PHOSPHATASES

Background: The bacterial phosphoenolpyruvate-dependent phosphotransfe rase system (PTS) mediates the energy-driven uptake of carbohydrates a nd their concomitant phosphorylation, In addition, the PTS is intimate ly involved in the regulation of a variety of metabolic and transcript ional processes in the bacterium. The multiprotein PTS consists of a m embrane channel and at least four cytoplasmic proteins or protein doma ins that sequentially transfer a phosphoryl group from phosphoenolpyru vate to the transported carbohydrate, Determination of the three-dimen sional structure of the IIB enzymes within the multiprotein complex wo uld provide insights into the mechanisms by which they promote efficie nt transport by the membrane channel IIC protein and phosphorylate the transported carbohydrate on the inside of the cell, Results: The crys tal structure of the IIB enzyme specific for cellobiose, IIBcellobiose (molecular weight 11.4 kDa), has been determined to a resolution of 1 .8 Angstrom and refined to an R factor of 18.7% (R(free) of 24.1%). Th e enzyme consists of a single four-stranded parallel beta sheet flanke d by helices on both sides. The phosphorylation site (Cys10) is locate d at the C-terminal end of the first beta strand, No positively charge d residues, which could assist in phosphoryl-transfer, can be found in or near the active site. The fold of IIBcellobiose is remarkably simi lar to that of the mammalian low molecular weight protein tyrosine pho sphatases. Conclusions: A comparison between IIBcellobiose and the str ucturally similar low molecular weight protein tyrosine phosphatases p rovides insight into the mechanism of the phosphoryltransfer reactions in which IIBcellobiose is involved, The differences in tertiary struc ture and active-site composition between IIBcellobiose and the glucose -specific IIBglucose give a structural explanation why the carbohydrat e-specific components of different families cannot complement each oth er.