Carbohydrate Transport by Group Translocation: The Bacterial Phosphoenolpyruvate: Sugar Phosphotransferase System

The Bacterial Phosphoenolpyruvate (PEP)Phosphoenolpyruvate (PEP) : Sugar Phosphotransferase System (PTS)Phosphotransferase System (PTS) mediates the uptake and phosphorylationPhosphorylation of carbohydrates, and controls the carbon- and nitrogen metabolism in response to the availability of sugars. PTS occur in eubacteria and in a few archaebacteria but not in animals and plants. All PTS comprise two cytoplasmic phosphotransferase proteinsHPr (EI and HPr)Enzyme I (EI) and a species-dependent, variable number of sugar-specific enzyme IIEnzyme II (IIABCD) complexes (IIA, IIB, IIC, IID). EI and HPr transfer phosphorylgroups from PEP to the IIA units. Cytoplasmic IIA and IIB units sequentially transfer phosphates to the sugar, which is transported by the IIC and IICIID integral membrane protein complexes. Phosphorylation by IIB and translocation by IIC(IID) are tightly coupled. The IIC(IID) sugar transporters of the PTS are in the focus of this review. There are four structurally different PTS transporter superfamilies (glucose, glucitol, ascorbate, mannose)Glucose (Glc) . Crystal structures are available for transporters of two superfamilies: bcIICmal (MalT, 5IWS, 6BVG) and bcIICchb (ChbC, 3QNQ) of B. subtilis from the glucose family, and IICasc (UlaA, 4RP9, 5ZOV) of E. coli from the ascorbate superfamilySuperfamily . They are homodimers and each protomer has an independent transportTransport pathway which functions by anElevator elevator-type alternating-accessAlternating access mechanism. bcIICmal and bcIICchb have the same fold, IICasc has a completely different fold. Biochemical and biophysical data accumulated in the past with the transporters for mannitol (IICBAmtl)Mannitol (Mtl) and glucose (IICBglc) are reviewed and discussed in the context of the bcIICmal crystal structures. The transporters of the mannose superfamily are dimers of protomers consisting of a IIC and a IID protein chain. The crystal structure is not known and the topology difficult to predict. Biochemical data indicate that the IICIID complex employs a different transportTransport mechanismTransport mechanism . Species specific IICIID serve as a gateway for the penetration of bacteriophageBacteriophage lambda DNA across, and insertion of class IIa bacteriocinsBacteriocin into the inner membrane. PTS transporters are inserted into the membrane by SecYEG translocon and have specific lipid requirements. Immunoelectron- and fluorescenceFluorescence microscopyMicroscopy indicate a non-random distribution and supramolecular complexes of PTS proteins.