CRYSTAL-STRUCTURE OF THE IIB SUBUNIT OF A FRUCTOSE PERMEASE (IIBLEV) FROM BACILLUS-SUBTILIS

The bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) mediates both the uptake of carbohydrates across the cytoplasmic membrane and their phosphorylation. During this process, a phosphoryl group is transferred from phosphoenolpyruvate via the general PTS pro teins enzyme I, HPr and the sugar-specific components IIA, IIB to the transported sugar. The crystal structure of the IIB subunit of a fruct ose transporter from Bacillus subtilis (IIBLev) was solved by MIRAS to a resolution of 2.9 Angstrom. IIBLev comprises 163 amino acid residue s that are folded into an open, mainly parallel beta-sheet with helice s packed on either face. The phosphorylation site (His15) is located o n the first loop (1/A) at one of the topological switch-points of the fold. Despite different global folds, IIBLev and HPr have very similar active-site loop conformations with the active-site histidine residue s located close to the N terminus of the first helix. This resemblance may be of functional importance, since both proteins exchange a phosp horyl group with the same IIA subunit. The structural basis of phospho ryl transfer from HPr to IIA(Man) to IIBMan was investigated by modeli ng of the respective transition state complexes using the known HPr an d IIA(Man) structures and a homology model of IIBMan that was derived from the IIBLev structure. All three proteins contain a helix that app ears to be suitable for stabilization of the phospho-histidine by dipo le and H-bonding interactions. Smooth phosphoryl transfer from one N-c ap position to the other appears feasible with a minimized transition state energy due to simultaneous interactions with the donor and the a cceptor helix. (C) 1998 Academic Press Limited.