Author contributions: P. Reviewers included: K. Carbohydrate uptake in many bacteria is regulated by the phosphotransferase protein IIA Glcenabling cells to use glucose preferentially over other sugars. Lactose permease LacY is one of many sugar permeases regulated by IIA Glcbut the mechanism of inducer exclusion is unclear.
We now show by isothermal titration calorimetry that IIA Glc binds to purified LacY with a stoichiometry of one, and that the interaction is driven by favorable solvation entropy. IIA Glc binding inhibits conformational dynamics of LacY and decreases binding affinity for sugar in a manner similar to that observed for melibiose permease MelB.
However, the thermodynamic mechanism by which the inhibitory effect is expressed differs for the two permeases. In a variety of bacteria, the phosphotransferase protein IIA Glc plays a key regulatory role in catabolite repression in addition to its role in the vectorial phosphorylation of glucose catalyzed by the phosphoenolpyruvate:carbohydrate phosphotransferase system PTS. Upon IIA Glc binding, the conformational entropy of LacY is restrained, which leads to a significant decrease in sugar affinity.
By suppressing conformational dynamics, IIA Glc blocks inducer entry into cells and favors constitutive glucose uptake and utilization. Furthermore, the studies support the notion that sugar binding involves an induced-fit mechanism that is inhibited by IIA Glc binding. Carbohydrate uptake in bacteria is catalyzed by a collection of sugar permeases that belong to different families of transport proteins.
In Escherichia and Salmonellathe phosphoenolpyruvate:carbohydrate phosphotransferase system PTS carries out both catalytic and regulatory functions and plays a key role in catabolite repression resulting in preferential utilization of glucose a constitutive process that is transported by vectorial phosphorylation catalyzed by the PTS 1 — 5.
The phosphotransferase protein IIA Glc plays a direct role in this regulation of inducible transport systems. Both LacY and MelB catalyze electrogenic symport of a galactoside with a cation 14 — Expression of the structural genes requires the participation of both a global transcriptional activator the cAMP—CAP complex and a specific inducer lactose or melibiose, respectively 322 Furthermore, IIA Glc binding reduces sugar affinity and the conformational entropy of MelB Stresulting in a block of melibiose entry into the cell, thus preventing induction of the mel operon.
The crystal structures of LacY 1432 — 36 and MelB St 37 both exhibit N- and C-terminal domains composed of six largely irregular transmembrane helices positioned pseudosymmetrically and surrounding a deep aqueous cavity Fig. Both permeases are members of the major facilitator superfamily. WT MelB St was crystallized in two outward periplasmic -open conformations with a sealed cytoplasmic side WT LacY was crystallized in an inward cytoplasmic -open conformation Recently, a LacY conformational mutant was captured in an almost occluded, narrowly outward-open conformation with a completely liganded galactoside molecule in the middle of the protein Evidence was also presented indicating that sugar binding involves induced fit, which causes LacY to transition from an open state to an occluded intermediate.
Lac Operon: Mechanism and Regulation
Crystal structures. All of the depicted structural models are on the same scale. The N- and C-terminal domains of both permeases are shown in different colors, and their middle cytoplasmic loops are colored yellow.
LacY and MelB St are in an inward-open and partially outward-open conformation, respectively. It is generally believed that a crystal structure typically captures a molecule in the lowest free-energy state.
With regard to LacY, abundant, independent lines of evidence demonstrate that the inward-open conformation in the absence of bound substrate represents the lowest free-energy state in both the membrane-embedded and detergent-solubilized state With both permeases, it has been well-documented that ligand binding induces conformational changes 394143 The results indicate that IIA Glc binding inhibits conformational entropy in both transporters, but by different energetic mechanisms.
The binding affinity is in a range similar to that obtained by direct binding with I-labeled IIA Glc 10as well as the affinity obtained with MelB St C and D Data fitting. Data are presented in Figs. Under the same experimental conditions without sugar, endothermic binding is also observed Fig. A best fit of the data suggests a K d of 6. The finding stands in opposition to previous conclusions that IIA Glc binding requires binding of substrate 1027The lac operon is a well-known example of an inducible gene network which regulates the transport and metabolism of lactose in Escherichia coli.
It encodes the genes for internalization of extracellular lactose and then its conversion to glucose. The lactose operon of E. The lac operon consists of a promoter P and operator O region followed by three structural genes lacZ, lacY, and lacA in the downstream.
A regulatory gene lacI I preceding the lac operon is responsible for producing a repressor R protein. In addition to structural genes, the lac operon also contains a number of regulatory DNA sequences. These are regions of DNA to which particular regulatory proteins can bind, controlling transcription of the operon. The promoter is the binding site for RNA polymerase, the enzyme that performs transcription. This mRNA is translated to give three protein products shown in the table below.
Prokaryotic genes expression is very often controlled by extracellular signals i. Two regulatory proteins communicate these signals with the genes:. Since its discovery, lac operon has been serving as a model system for understanding different aspects of gene regulations.
The lac repressor senses the presence of lactose more precisely allolactose-an isomer of lactose in the medium. In the absence of allolactose A the repressor protein R binds to the operator region O and blocks the RNA polymerase from transcribing the structural genes. When lactose is present in a medium, some of it will be converted to allolactose. This allolactose binds to the repressor protein. Binding of allolactose with repressor protein changes the shape of repressor protein so it can no longer binds to the operator region.
Once the mRNA is produced, it is translated to produce enzymes. Enzyme for lactose utilization can be induced by adding lactose in the growth medium but, it would be wasteful to induce these enzymes if the cells are already growing on a carbon source that they could use more efficiently e.
This regulation is governed by a phenomenon called catabolite repressionalso known as glucose effect. Catabolite repression ensures that the cells use the BEST carbon source first.
The gene encoding CAP is located elsewhere on the bacterial chromosome, not linked to the lac genes. Cyclic AMP is synthesized from ATP by an enzyme called adenylate cyclase, and glucose inhibits the synthesis of cyclic AMP and stimulates its transport out of the cell. When glucose is transported into the cell, the cyclic AMP level in the cell is lowered.
Structure and mechanism of the lactose permease of Escherichia coli
This will cause loose binding of RNA polymerase to the promoter region resulting low level of transcription. When there is no any cyclic AMP left in the cell, the transcription will be ceased.
In this case, transcription occurs only at a low level. Each of the regulatory proteins CAP and lac repressor responds to one environmental signal and communicates it to the lac genes. The combined effect of these two regulators ensures that the genes are expressed at significant levels only when lactose is present and glucose is absent.Membrane transport proteins that transduce free energy stored in electrochemical ion gradients into a concentration gradient are a major class of membrane proteins.
We report the crystal structure at 3. The molecule is composed of N- and C-terminal domains, each with six transmembrane helices, symmetrically positioned within the permease. A large internal hydrophilic cavity open to the cytoplasmic side represents the inward-facing conformation of the transporter.
The structure with a bound lactose homolog, beta-D-galactopyranosylthio-beta-D-galactopyranoside, reveals the sugar-binding site in the cavity, and residues that play major roles in substrate recognition and proton translocation are identified.
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Macromolecule Content Total Structure Weight: Structure and mechanism of the lactose permease of Escherichia coli Abramson, J.
Hide Full Abstract. Reference Sequence. View more in-depth experimental data. Escherichia coli. Membrane protein Mpstruc. Protein Feature View Expand.Lactose Permease PDB entry 1pv7 is a transmembrane protein that facilitates the passage of lactose across the phospholipid bi-layer of the cell membrane.
Fucose permease transports fucose across cell membrane. Lactose is a disaccharide carbohydrate found primarily in mammalian milk. The disaccharide consists of the monosacharides glucose and galactose. When the lactose is ingested and absorbed into the cell, the enzyme lactase breaks the disaccharide into its monosaccharide subunits. These are in turn used in the cellular respiration process and broken down further into energy for the cell. Lactose permease belongs to the family of so called Major Facilitators.
There are six sidechains that play an irreplaceable role in the active transport of lactose through the protein. Three of these sidechains, and have been shown to be crucial in substrate binding activities. Additionally, there are two residues that are suspected to play an important role in the alignment of the galactopyranosyl end of the substrate.
These are and. These sidechains, which make up the active site of the protein, can be found within the large internal of the lactose permease. It is here where the is received for transport and it is the location from which it is deposited into the cell. The currently crystalized form of the permease is considered an 'inward-facing' conformation. This implies that the hydrophilic cavity mentioned previously is positioned with the opening towards the cytoplasm of the cell.
Conversely, and outward-facing conformation would have the cavity facing the periplasm. Category : Topic Page. Lactose Permease From Proteopedia. Jump to: navigationsearch. Show: Asymmetric Unit Biological Assembly.
Export Animated Image. Views Article Discussion Edit this page History. Toolbox Upload file Special pages Printable version Permanent link.Related to permease: transacetylase.
Any of several cell-membrane proteins that function as channels for the passive transport of a specific molecule in or out of the cell.
Published by Houghton Mifflin Company. All rights reserved. Any of a group of membrane-bound carriers enzymes that cause transport of solute through a semipermeable membrane. Collins Dictionary of Biology, 3rd ed. Hale, V. Saunders, J. Margham Mentioned in?VMD Series, Lactose Transport in Lactose Permease Membrane Protein
References in periodicals archive? The major permease responsible for high-affinity phenylalanine transport is Bap2p, rather than the broad-specificity permeaseAgp1p Figure 1. Bio-recognition and functional lipidomics by glycosphingolipid transfer technology. Mapping of the functional domains of TOR, an adaptor protein regulating protein trafficking in Leishmania. This dual role would be analogous to the sugar permease BglF in Escherichia coli and the yeast ammonium transporter Mep2p.
Understanding and improving salt tolerance in plants. Alzheimer disease and other dementias. Under most growing conditions it appears that organic anions are excreted via membrane-bound ' permease ' proteins.
Progress in selected areas of rhizosphere research on P acquisition. Based on genetic and biochemical study, the lactose permease of Escherichia coli is a model system for this research. The effect of charged amino acid mutations in hydrophobic transmembrane domain III in the lactose permease of Escherichia coli.Systems used to automatically annotate proteins with high accuracy:.
Select item s and click on "Add to basket" to create your own collection here entries max. Manual assertion based on experiment in i. Manual assertion inferred by curator from i. Manual assertion based on opinion in i. Manual assertion inferred from combination of experimental and computational evidence i. You are using a version of browser that may not display all the features of this website. Please consider upgrading your browser.
Your basket is currently empty. Note that the 'protein existence' evidence does not give information on the accuracy or correctness of the sequence s displayed. Select a section on the left to see content. Can transport lactose, melibiose, lactulose or the analog methylthio-beta,D-galactopyranoside TMGbut not sucrose or fructose PubMed:PubMed:PubMed: The substrate specificity is directed toward the galactopyranosyl moiety of the substrate PubMed: Alternative name s :.
Name: lacY 1 Publication Manual assertion based on opinion in i Ref. Ordered Locus Names: bJW The information is filed in different subsections. Length: Mass Da : 46, It is useful for tracking sequence updates.
The algorithm is described in the ISO standard. Molecular mass is Da. Determined by ESI. Escherichia sp. Full view. Escherichia coli DMembrane transport proteins transduce free energy stored in electrochemical ion gradients into a concentration gradient and are a major class of membrane proteins, many of which play important roles in human health and disease.
Recently, the X-ray structure of the Escherichia coli lactose permease LacYan intensively studied member of a large group of related membrane transport proteins, was solved at 3.
LacY is composed of N- and C-terminal domains, each with six transmembrane helices, symmetrically positioned within the molecule. The structure represents the inward-facing conformation, as evidenced by a large internal hydrophilic cavity open to the cytoplasmic side.
The structure with a bound lactose homolog reveals the sugar-binding site in the cavity, and a mechanism for translocation across the membrane is proposed in which the sugar-binding site has alternating accessibility to either side of the membrane.
Abstract Membrane transport proteins transduce free energy stored in electrochemical ion gradients into a concentration gradient and are a major class of membrane proteins, many of which play important roles in human health and disease. Publication types Research Support, Non-U. Gov't Research Support, U.