Biophysical studies of the membrane-embedded and cytoplasmic forms of the glucose-specific Enzyme II of the E. coli phosphotransferase system (PTS)

Protein-Protein Interactions in the Cytoplasmic Membrane of Escherichia coli: Influence of the Overexpression of Diverse Transporter-Encoding Genes on the Activities of PTS Sugar Uptake Systems

The prokaryotic phosphoenolpyruvate (PEP):sugar phosphotransferase system (PTS) concomitantly transports and phosphorylates its substrate sugars. In a recent publication, we provided evidence that protein-protein interactions of the fructose-specific integral membrane transporter (FruAB) with other PTS sugar group translocators regulate the activities of the latter systems in vivo and sometimes in vitro. In this communication, we examine the consequences of the overexpression of several different transport systems on the activities of selected PTS and non-PTS permeases. We report that high levels of these transport systems enhance the in vivo activities of several other systems in a fairly specific fashion. Thus, (1) overexpression of ptsG (glucose porter) selectively enhanced mannitol, N-acetylglucosamine, and 2-deoxyglucose (2DG) uptake rates; (2) overexpression of mtlA (mannitol porter) promoted methyl α-glucoside (αMG) and 2DG uptake; (3) manYZ (but not manY alone) (mannose porter) overexpression enhanced αMG uptake; (4) galP (galactose porter) overexpression enhanced mannitol and αMG uptake; and (5) ansP (asparagine porter) overexpression preferentially enhanced αMG and 2DG uptake, all presumably as a result of direct protein-protein interactions. Thus, it appears that high level production of several integral membrane permeases enhances sugar uptake rates, with the PtsG and ManXYZ systems being most consistently stimulated, but the MtlA and NagE systems being more selectively stimulated and to a lesser extent. Neither enhanced expression nor in vitro PEP-dependent phosphorylation activities of the target PTS systems were appreciably affected. The results are consistent with the suggestion that integral membrane transport proteins form an interacting network in vivo with physiological consequences, dependent on specific transporters and their concentrations in the membrane.

Protein:Protein interactions in the cytoplasmic membrane apparently influencing sugar transport and phosphorylation activities of the e. coli phosphotransferase system

The multicomponent phosphoenolpyruvate (PEP)-dependent sugar-transporting phosphotransferase system (PTS) in Escherichia coli takes up sugar substrates from the medium and concomitantly phosphorylates them, releasing sugar phosphates into the cytoplasm. We have recently provided evidence that many of the integral membrane PTS permeases interact with the fructose PTS (FruA/FruB) [1]. However, the biochemical and physiological significance of this finding was not known. We have carried out molecular genetic/biochemical/physiological studies that show that interactions of the fructose PTS often enhance, but sometimes inhibit the activities of other PTS transporters many fold, depending on the target PTS system under study. Thus, the glucose (Glc), mannose (Man), mannitol (Mtl) and N-acetylglucosamine (NAG) permeases exhibit enhanced in vivo sugar transport and sometimes in vitro PEP-dependent sugar phosphorylation activities while the galactitol (Gat) and trehalose (Tre) systems show inhibited activities. This is observed when the fructose system is induced to high levels and prevented when the fruA/fruB genes are deleted. Overexpression of the fruA and/or fruB genes in the absence of fructose induction during growth also enhances the rates of uptake of other hexoses. The β-galactosidase activities of man, mtl, and gat-lacZ transcriptional fusions and the sugar-specific transphosphorylation activities of these enzyme transporters were not affected either by frustose induction or by fruAB overexpression, showing that the rates of synthesis of the target PTS permeases were not altered. We thus suggest that specific protein-protein interactions within the cytoplasmic membrane regulate transport in vivo (and sometimes the PEP-dependent phosphorylation activities in vitro) of PTS permeases in a physiologically meaningful way that may help to provide a hierarchy of preferred PTS sugars. These observations appear to be applicable in principle to other types of transport systems as well.

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

The Bacterial Phosphoenolpyruvate (PEP) : Sugar Phosphotransferase System (PTS) mediates the uptake and phosphorylation 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 proteins (EI and HPr) and a species-dependent, variable number of sugar-specific enzyme II 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) . Crystal structures are available for transporters of two superfamilies: bcIIC^mal (MalT, 5IWS, 6BVG) and bcIIC^chb (ChbC, 3QNQ) of B. subtilis from the glucose family, and IIC^asc (UlaA, 4RP9, 5ZOV) of E. coli from the ascorbate superfamily . They are homodimers and each protomer has an independent transport pathway which functions by an elevator-type alternating-access mechanism. bcIIC^mal and bcIIC^chb have the same fold, IIC^asc has a completely different fold. Biochemical and biophysical data accumulated in the past with the transporters for mannitol (IICBA^mtl) and glucose (IICB^glc) are reviewed and discussed in the context of the bcIIC^mal 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 transport mechanism . Species specific IICIID serve as a gateway for the penetration of bacteriophage lambda DNA across, and insertion of class IIa bacteriocins into the inner membrane. PTS transporters are inserted into the membrane by SecYEG translocon and have specific lipid requirements. Immunoelectron- and fluorescence microscopy indicate a non-random distribution and supramolecular complexes of PTS proteins.

The involvement of transport proteins in transcriptional and metabolic regulation

Transport proteins have sometimes gained secondary regulatory functions that influence gene expression and metabolism. These functions allow communication with the external world via mechanistically distinctive signal transduction pathways. In this brief review we focus on three transport systems in Escherichia coli that control and coordinate carbon, exogenous hexose-phosphate and phosphorous metabolism. The transport proteins that play central roles in these processes are: first, the phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS), second, the glucose-6-phosphate receptor, UhpC, and third, the phosphate-specific transporter, PstSABC, respectively. While the PTS participates in multiple complex regulatory processes, three of which are discussed here, UhpC and the Pst transporters exemplify differing strategies.

Lipid dependencies, biogenesis and cytoplasmic micellar forms of integral membrane sugar transport proteins of the bacterial phosphotransferase system

Permeases of the prokaryotic phosphoenolpyruvate-sugar phosphotransferase system (PTS) catalyse sugar transport coupled to sugar phosphorylation. The lipid composition of a membrane determines the activities of these enzyme/transporters as well as the degree of coupling of phosphorylation to transport. We have investigated mechanisms of PTS permease biogenesis and identified cytoplasmic (soluble) forms of these integral membrane proteins. We found that the catalytic activities of the soluble forms differ from those of the membrane-embedded forms. Transport via the latter is much more sensitive to lipid composition than to phosphorylation, and some of these enzymes are much more sensitive to the lipid environment than others. While the membrane-embedded PTS permeases are always dimeric, the cytoplasmic forms are micellar, either monomeric or dimeric. Scattered published evidence suggests that other integral membrane proteins also exist in cytoplasmic micellar forms. The possible functions of cytoplasmic PTS permeases in biogenesis, intracellular sugar phosphorylation and permease storage are discussed.