Lactose transporter

The lactose transporter (lac permease) of E. coli is without doubt the most intensively studied and best understood of the bacterial proton-linked sugar transporters. Since its sequence was reported in 1980 [233] prodigious efforts have been made to elucidate its molecular mechanism by site-directed mutagenesis and other means. These studies have recently been reviewed elsewhere [234,235] and so will not be discussed in detail here. The important question for the present Chapter is whether the protein is related to the sugar-transporter family and so has lessons to teach us about their mechanisms. The permease is a 417-residue protein, and, like the other... 

The lactose transporter is one member of the major facilitator superfamily (MFS) of transporters, which comprises 28 families. Almost all proteins in this superfamily have 12 transmembrane domains (the few exceptions have 14). The proteins share rela-... 

FIGURE 11-43 Structure of the lactose transporter (lactose permease) of E. coli. (a) Ribbon representation viewed parallel to the plane of the membrane shows the 12 transmembrane helices arranged in two nearly symmetrical domains shown in different shades of blue. In the form of the protein for which the crystal structure was determined, the substrate sugar (red) is bound near the middle of the membrane where it is exposed to the cytoplasm (derived from PDB ID 1 PV7). (b) The structural changes postulated to take place during one transport... 

How is proton movement into the cell coupled with lactose uptake Extensive genetic studies of the lactose transporter have established that of the 417 residues in the protein, only 6 are absolutely essential for cotransport of H+ and lactose—some for lactose binding, others for proton transport. Mutation in either of two residues (Glu325 and Arg302 Fig. 11-43) results in a protein still able to catalyze facilitated diffusion of lactose... 

Surface Density of a Membrane Protein E. coli can be induced to make about 10,000 copies of the lactose transporter (Mr 31,000) per cell. Assume that E. coli is a cylinder 1 /am in diameter and 2 /am long. What fraction of the plasma membrane surface is occupied by the lactose transporter molecules Explain how you arrived at this conclusion. 

Includes the newly solved structure of the lactose transporter of E. coli. 

Calmes, R. 1978. Involvement of phosphoenolpyruvate in the catabolism of caries-conducive disaccharides by Streptococcus mutans Lactose transport. Infect. Immun. 19, 934-942. 

The bacterial lactose-transport protein (lactose permease) transports j3-galactosides, such as lactose, o-nitrophenyl-jS-galactoside, and isopropyl-jS-thiogalactoside. It does not transport galactosides with an a-glycosidic linkage. 

Membrane vesicles of E. coli that possess the lactose permease are preloaded with KC1 and are suspended in an equal concentration of NaCl. It is observed that these vesicles actively, although transiently, accumulate lactose if valinomycin is added to the vesicle suspension. No such active uptake is observed if KC1 replaces NaCl in the suspending medium. Explain these results in light of what you know about the mechanism of lactose transport and the properties of valinomycin. 

Newman, M.J. Wilson, T.H. (1980). Solubilization and reconstitution of the lactose transport system from Escherichia coli. J. Biol. Chem. 255,10583-10586. 

In bacteria, Na+ is often replaced by H+ as the carrier of nutrients. The best known mechanisms of this sort are the various galactoside (e.g., lactose) transporters, because lactose is an important carbon source for many bacteria. The work that can be performed (e.g., in transporting a nutrient) by generating a proton gradient across membranes may be expressed by the following, which encompasses both the chemical component (concentration gradient) and the electrical component in the form of the membrane potential AW ... 

The rate-limiting step in lactose transport (in E. coli) is A. Binding of a H+ outside the cell... 

Lactose transporter/permease of coli 2.A.I.I. Sugar porter family... 

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