GalP (protein)

Last updated June 21, 2022

The galactose permease or GalP found in Escherichia coli is an integral membrane protein involved in the transport of monosaccharides, primarily hexoses, for utilization by E. coli in glycolysis and other metabolic and catabolic pathways (3,4). It is a member of the Major Facilitator Super Family (MFS) and is homologue of the human GLUT1 transporter (4). Below you will find descriptions of the structure, specificity, effects on homeostasis, expression, and regulation of GalP along with examples of several of its homologues.

Structure

Galactose Permease (GalP), is a member of the Major Facilitator Super Family (MFS) and therefore has structural similarities to the other members of this super family such as GLUT1 (4). All members of the MFS have 12 membrane spanning alpha(α)-helices with both the C- and N-termini located on the cytoplasmic side of the membrane (4). Figure 1a (3) depicts how the 12 helices are divided into two halves, that are pseudo-symmetric, of 6 helices which are attached by a long hydrophilic cytoplasmic loop between helix 6 and helix 7 (2,3,4). These two halves come together to form a pore for substrate transport, in GalP, the substrates are primarily galactose, glucose, and H+. GalP monomers have a pore of approximately 10Å in diameter, which is consistent with the pore sizes found in other members of the MFS, between 10-15Å (4). GalP has been found as an oligomer formed by a homotrimer of GalP monomers that exhibits p3 or 3-fold rotational symmetry (Figure 1b-c) (4). GalP is the first member of the MFS that has been found as a trimer and to be biologically active in its trimeric form; it is thought that the GalP oligomer is formed for stability (4).

Specificity

GalP is a monosaccharide transporter that uses a chemiosmotic mechanism to transport its substrates into the cytoplasm of E. coli (1). Glucose, galactose and other hexoses are transported by GalP by the use of the proton gradient produced by the electron transport chain and reversible ATPase (1). GalP can bind specifically to the hexoses with preferential binding of galactose and glucose through the pores in each monomer (2,3). It transports these sugars at faster rates with a proton gradient but can still transport them in a leaky fashion without a proton gradient present (4). As stated before GalP shares similarities with GLUT1 and other members of the MFS and like GLUT1, GalP can be inhibited by the antibiotics cytochalasin B and forskolin (Figure 1a) (3), which competitively bind to the pore blocking sugar transport into the cell (2,3,4). Forskolin is a structural homologue of D-galactose (Figure 1a) (3) and therefore can bind with a similar affinity to the transporter. Cytochalasin B may bind to an asparagine residue (Asn394) in the pore, blocking saccharide uptake, which is also found in the GLUT1 transporter (2,3). GalP can transport lactose or fructose but with low affinity, only allowing these sugars to "leak" across the membrane when glucose, galactose, or other hexoses aren't present for transport (4).

Homeostasis

The GalP symporter links galactose and proton import, using the favorable proton concentration gradient to move galactose against its concentration gradient. However, this mechanism, if in isolation, would result in acidification of the cytoplasm and cessation of galactose import(14). To prevent this, E. coli utilizes ion pumps designed to raise intracellular pH (13,14). During electron transport (a key step in ATP production in respiration), energy harnessed from electrons is used to pump protons into the periplasmic space to build a proton motive force. Primary proton pumps, responsible for pumping protons out of the cytoplasm, can be active without the synthesis of ATP and are the primary mechanism through which protons are exported (13,14). Coupling galactose/proton import with proton export would maintain pH homeostasis. As protons are charged molecules, their import or export could disrupt the membrane potential of the cell (14). However, simultaneous import and export of protons would result in no change in the net charge of the cell, thus no net change in membrane potential.

Regulation/Expression

The GalP/H+ symporter is the galactose permease from the galP gene of the Escherichia coli genome. Galactose is an alternate carbon source to the preferable glucose . The cAMP/CRP catabolite repression regulator is most likely involved in the regulation of GalP expression (Figure 2) (9). The two proteins responsible for inhibiting transcription from the gal regulon are GalR and GalS (Figure 4) (11). GalR and GalS have very similar primary structure sequences, and have the same binding sites on the operator (11). In the presence of D-galactose, GalR and GalS are inhibited since they are repressors (5, 11). However, when GalP is not required (i.e. when glucose is available), GalR/GalS will bind the promoter operator site thus blocking transcription and preventing cAMP-CRP activation (11). GalS is seen to bind only in the presence of GalR, so both of these proteins are required for repression (11). cAMP is what modulates CRP at the promoter. The cAMP-CRP complex activates the gal regulon and is responsible for upregulation of GalP (Figure 2) (9,11). GalP is also repressed in the presence of glucose since the cell will prefer glucose over galactose (7).

There is also a study on the involvement of NagC in regulation, a protein from the nagC gene which is responsible for N-acetylglucosamine repression (5). This study suspects that NagC cooperates with GalR and GalS by binding to a single-high affinity site upstream of the galP promoter as well in order to suppress gal regulon transcription (5).

Other Bacteria Symporters

Several other symporters have been identified in E. coli and in other bacteria. E. coli has a well-studied GltS glutamate/Na+ symporter that aids in the uptake of glutamate into the cell along with an influx of sodium ions. It also has a serine-threonine symporter, SstT, that also uses an influx of sodium ions for solute uptake.

A Na+/glucose symporter (SglT) has been identified in Vibrio parahaemolyticus (10). Sodium ions induced the cells’ uptake of glucose in a study of phosphotransferase-system (PTS) mutants (10). Clostridium difficile has a symporter homologous to that of the V. parahaemolyticus SglT (6). A citrate/Na+ symporter, CitS, seems to be common between Vibrio cholerae, Salmonella Typhi, and Klebsiella pneumoniae (6). This symporter uses the influx of sodium ions in order to bring citrate into the cell, which is an important substrate to have for metabolic processes such as decarboxylation of oxaloacetate (6). A H+/amino acid symporter BrnQ can be found in Lactobacillus delbruckii, and Pseudomonas aeruginosa has the BraB symporter for substrates such as glutamate as well (6).

Solute/ion symporters are very commonly found in bacteria since they are very important. Homeostasis and regulated uptake for metabolic pathways is essential for bacterial survival.

GLUT-1: A Eukaryotic Homolog

GalP is homologous to GLUT-1 found in mammalian cells (12). Both transporters are MFS transporters and possess 29% sequence identity (4). GLUT-1 is a glucose transporter present in most mammalian cells (Figure 5) (12). Its structure is nearly identical to that of GalP – possessing cytoplasmic amino and carboxy termini, twelve membrane spanning α helices, a periplasmic glycosylation site between helices 1 and 2, and a cytoplasmic α-helix loop between helices 6 and 7 (12). GLUT-1 ranges from 45 to 55 kDa; the size variation depends upon the extent of glycosylation (12).

While GLUT-1 is found in most mammalian cells, certain tissue types express this transporter more so than others. GLUT-1 is expressed in high levels on erythrocytes, embryonic cells, fibroblasts, and endothelial cells (12). GLUT-1 is also one of the main transporters involved in transporting glucose across the blood brain barrier (12).

Generally, GLUT-1 acts as a facilitative transporter of glucose, transporter glucose along its concentration gradient. When glucose binds to GLUT-1, it stimulates a conformational change, allowing glucose to be released on the opposite side of the membrane (4,12). GLUT-1 is a bidirectional transporter and possesses glucose binding sites accessible on both the cytoplasmic and extracellular faces (4,12). On the rare occasion that GLUT-1 transports glucose against its concentration gradient, Glut-1 uses an energy source, typically ATP, to move the glucose. Like GalP, GLUT-1 is inhibited via the binding of cytochalasin B and forskolin (12).

Related Research Articles

In cellular biology, active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses adenosine triphosphate (ATP), and secondary active transport that uses an electrochemical gradient.

Reuptake is the reabsorption of a neurotransmitter by a neurotransmitter transporter located along the plasma membrane of an axon terminal or glial cell after it has performed its function of transmitting a neural impulse.

The lactose operon is an operon required for the transport and metabolism of lactose in E. coli and many other enteric bacteria. Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available through the activity of beta-galactosidase. Gene regulation of the lac operon was the first genetic regulatory mechanism to be understood clearly, so it has become a foremost example of prokaryotic gene regulation. It is often discussed in introductory molecular and cellular biology classes for this reason. This lactose metabolism system was used by François Jacob and Jacques Monod to determine how a biological cell knows which enzyme to synthesize. Their work on the lac operon won them the Nobel Prize in Physiology in 1965.

Mediated transport Transportation of substances via membrane

Mediated transport refers to transport mediated by a membrane transport protein. Substances in the human body may be hydrophobic, electrophilic, contain a positively or negatively charge, or have another property. As such there are times when those substances may not be able to pass over the cell membrane using protein-independent movement. The cell membrane is imbedded with many membrane transport proteins that allow such molecules to travel in and out of the cell. There are three types of mediated transporters: uniport, symport, and antiport. Things that can be transported are nutrients, ions, glucose, etc, all depending on the needs of the cell. One example of a uniport mediated transport protein is GLUT1. GLUT1 is a transmembrane protein, which means it spans the entire width of the cell membrane, connecting the extracellular and intracellular region. It is a uniport system because it specifically transports glucose in only one direction, down its concentration gradient across the cell membrane.

A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans. Collectively membrane transporters and channels are known as the transportome. Transportomes govern cellular influx and efflux of not only ions and nutrients but drugs as well.

An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in opposite directions, one into the cell and one out of the cell. Na⁺/H⁺ antiporters have been reviewed.

Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They enable cotransport and include antiporters and symporters. In general, cotransporters consist of two out of the three classes of integral membrane proteins known as transporters that move molecules and ions across biomembranes. Uniporters are also transporters but move only one type of molecule down its concentration gradient and are not classified as cotransporters.

Glucose transporter Family of monosaccharide transport proteins

Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose across the plasma membrane, a process known as facilitated diffusion. Because glucose is a vital source of energy for all life, these transporters are present in all phyla. The GLUT or SLC2A family are a protein family that is found in most mammalian cells. 14 GLUTS are encoded by human genome. GLUT is a type of uniporter transporter protein.

In biology, a transporter is a transmembrane protein that moves ions across a biological membrane to accomplish many different biological functions including, cellular communication, maintaining homeostasis, energy production, etc. There are different types of transporters including, pumps, uniporters, antiporters, and symporters. Active transporters or ion pumps are transporters that convert energy from various sources—including adenosine triphosphate (ATP), sunlight, and other redox reactions—to potential energy by pumping an ion up its concentration gradient. This potential energy could then be used by secondary transporters, including ion carriers and ion channels, to drive vital cellular processes, such as ATP synthesis.

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Efflux (microbiology) Protein complexes that move compounds, generally toxic, out of bacterial cells

All microorganisms, with a few exceptions, have highly conserved DNA sequences in their genome that are transcribed and translated to efflux pumps. Efflux pumps are capable of moving a variety of different toxic compounds out of cells, such as antibiotics, heavy metals, organic pollutants, plant-produced compounds, quorum sensing signals, bacterial metabolites and neurotransmitters via active efflux, which is vital part for xenobiotic metabolism. This active efflux mechanism is responsible for various types of resistance to bacterial pathogens within bacterial species - the most concerning being antibiotic resistance because microorganisms can have adapted efflux pumps to divert toxins out of the cytoplasm and into extracellular media.

Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron. They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron. If the plasma glucose concentration is too high (hyperglycemia), glucose passes into the urine (glucosuria) because SGLT are saturated with the filtered glucose.

A symporter is an integral membrane protein that is involved in the transport of two different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the cell membrane at the same time, and is, therefore, a type of cotransporter. The transporter is called a symporter, because the molecules will travel in the same direction in relation to each other. This is in contrast to the antiport transporter. Typically, the ion(s) will move down the electrochemical gradient, allowing the other molecule(s) to move against the concentration gradient. The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled with the active transport of the molecule(s).

Sodium/glucose cotransporter 1 (SGLT1) also known as solute carrier family 5 member 1 is a protein in humans that is encoded by the SLC5A1 gene which encodes the production of the SGLT1 protein to line the absorptive cells in the small intestine and the epithelial cells of the kidney tubules of the nephron for the purpose of glucose uptake into cells. Through the use of the sodium glucose cotransporter 1 protein, cells are able to obtain glucose which is further utilized to make and store energy for the cell.

The major facilitator superfamily (MFS) is a superfamily of membrane transport proteins that facilitate movement of small solutes across cell membranes in response to chemiosmotic gradients.

Lactose permease is a membrane protein which is a member of the major facilitator superfamily. Lactose permease can be classified as a symporter, which uses the proton gradient towards the cell to transport β-galactosides such as lactose in the same direction into the cell.

Members of the Solute:Sodium Symporter (SSS) Family (TC# 2.A.21) catalyze solute:Na⁺ symport. The SSS family is within the APC Superfamily. The solutes transported may be sugars, amino acids, organo cations such as choline, nucleosides, inositols, vitamins, urea or anions, depending on the system. Members of the SSS family have been identified in bacteria, archaea and eukaryotes. Almost all functionally well-characterized members normally catalyze solute uptake via Na⁺ symport.

Bacterial Leucine Transporter (LeuT) is a bundled twelve alpha helix protein which belongs to the family of transporters that shuttle amino acids in and out of bacterial cells. Specialized in small hydrophobic amino acids such as leucine and alanine, this transporter is powered by the gradient of sodium ions that is normally maintained by healthy cells across their membranes. LeuT acts as a symporter, which means that it links the passage of a sodium ion across the cell membrane with the transport of the amino acid in the same direction. It was first crystallized to understand the inner molecular mechanisms of antidepressant's work since it has a close resemblance with the human neurotransmitter transporters that these drugs block, thus inhibiting the reuptake of chemical messengers across the cell membrane of nerve axons and glial cells.

The potassium (K⁺) uptake permease (KUP) family (TC# 2.A.72) is a member of the APC superfamily of secondary carriers. Proteins of the KUP/HAK/KT family include the KUP (TrkD) protein of E. coli and homologues in both Gram-positive and Gram-negative bacteria. High affinity (20 μM) K⁺ uptake systems (Hak1, TC# 2.A.72.2.1) of the yeast Debaryomyces occidentalis as well as the fungus, Neurospora crassa, and several homologues in plants have been characterized. Arabidopsis thaliana and other plants possess multiple KUP family paralogues. While many plant proteins cluster tightly together, the Hak1 proteins from yeast as well as the two Gram-positive and Gram-negative bacterial proteins are distantly related on the phylogenetic tree for the KUP family. All currently classified members of the KUP family can be found in the Transporter Classification Database.

Major facilitator superfamily domain containing 6 like (MFSD6L) is a protein encoded by the MFSD6L gene in humans. The MFSD6L protein is a transmembrane protein that is part of the major facilitator superfamily (MFS) that uses chemiosmotic gradients to facilitate the transport of small solutes across cell membranes.

References

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14. Schweizer, H. (2011). Homeostasis. Lecture. 7 March 2011.