| 4.A.1 The PTS Glucose-Glucoside (Glc) Family
The Glc family includes porters specific for glucose, glucosamine, N-acetylglucosamine and a large variety of α- and β-glucosides. However, not all β-glucoside PTS porters are in this class, as the PTS porter first described (as the cellobiose (Cel) β-glucoside porter) is the diacetylchitobiose porter in the Lac family (TC #4.A.3). The IIA, IIB and IIC domains of all of the group translocators listed below are demonstrably homologous. These porters (the IIC domains) show limited sequence similarity with and are homologous to members of the Fru family (TC #4.A.2) and with (but less so) members of the Lac family (TC #4.A.3)and the Gut (glucitol) family (TC# 4.A.4). The IIC domains of the glucose (4.A.1.1) and glucoside (4.A.1.2) subfamilies are nearly as distant from each other as they are from the Fru, Mtl and Lac families. The Gut family is more distant due to an internal rearrangement relative to members of the other families mintioned. As is true of other members of the PTS-GFL superfamily, the IIC domains of these permeases probably have a uniform 10 TMS topology (McCoy et al. 2016; Vastermark and Saier 2016). Butanol toxicity in Clostridium acetobutylicum results in destruction of the PTS, thereby preventing glucose transport and phosphorylation (Gao et al. 2021).
Several of the PTS porters in the Glc family lack their own IIA domains and instead use the glucose IIA protein (IIAglc or Crr). Most of these porters have the B and C domains linked together in a single polypeptide chain. A cysteyl residue in the IIB domain is phosphorylated by direct phosphoryl transfer from IIAglc(his~P) or one of its homologues. Those porters which lack a IIA domain include the maltose (Mal), arbutin-salicin-cellobiose (ASC), trehalose (Tre), putative glucoside (Glv) and sucrose (Scr) porters of E. coli. Most, but not all Scr porters of other bacteria also lack their own IIA domains.
BglF consists of a transmembrane domain, which in addition to TMSs, contains a large cytoplasmic loop. According to Yagur-Kroll et al., 2009, this loop, connecting TMSI to TMSII, contains regions that alternate between facing-in and facing-out states and creates the sugar translocation channel. Yagur-Kroll et al., 2009 demonstrated spatial proximity between positions at the center of the big loop and the phosphorylation site, suggesting that these two regions come together to execute sugar phosphotransfer.
The three-dimensional structures of the IIA and IIB domains of the E. coli glucose porter have been elucidated. IIAglc has a complex β-sandwich structure while IIBglc is a split αβ-sandwich with a topology unrelated to the split αβ-sandwich structure of HPr. Some bacteria have many PTS transport systems belonging to different families. For example, the solventogenic Clostridium acetobutylicum ATCC 824 has 13 altogether with 6 in the Glc family, 2 in the Fru family, 2 in the Lac family, 1 in the Gat family and 2 in the Man family. However, Clostridium beijerinckii has 43 phosphotransferases (Mitchell 2015).
Structures of an N, N′-diacetylchitobiose EIIC transporter bcChbC ( 7) and a maltose EIIC transporter bcMalT, both from Bacillus cereus,
have been reported. bcChbC and bcMalT share 19% sequence identity and
50% similarity, and both have the same structural fold with almost
all of the secondary structural elements conserved ( Ren et al. 2018).
Both proteins are homodimers, and each protomer has 10 transmembrane sequenes
(TMSs 1-10), two reentrant loops (HP1-2), and two amphipathic
helices (AH1-2). These structural elements fold into two distinctive
structural domains. The dimerization domain (also referred to as the
interface domain), which consists of TMSs 1-5 and AH1, forms an expansive
dimer interface. The substrate-binding domain (also referred to as the
transport domain), which is composed of TMSs 6-10 and two reentrant loops
(HP1-2), contains the sugar-binding site. In both structures, the sugar
substrate is coordinated by residues from TMSs 6 and 7, HP1, and HP2. The two domains are bridged by an amphipathic helix (AH2).
The bcChbC and bcMalT structures represent different conformations required to complete a transport cycle.
Based on the location of the substrate-binding site, bcChbC is in an
inward-facing conformation, while bcMalT is in an outward-facing
conformation (Ren et al. 2018).
When the two structures are aligned by their dimerization domains, the substrate-binding domain can carry the substrate across
the membrane by a rigid-body motion. A similar elevator-type transport
mechanism has been reported in a number of secondary solute
transporters including amino acid transporters (EAAT1 (TC# 2/A/23/2/1) and GltPh (TC# 2.A.23.1.5)), bile acid transporters (ASBT; 2.A.28.1.2), proton sodium exchangers (NhaA: TC# 2.A.33.1.1), concentrative nucleotide transporters (CNTNW; TC# 2.A.41.2.6), and citrate transporters (vcINDY and SeCitS).
These transporters have different structural folds, and yet they all
transport substrates from one side of the cell membrane to the other by
rigid-body motions of a substrate-binding domain.
Although
by comparing the inward-facing bcChbC structure and the outward-facing
bcMalT structure one can postulate that the glucose superfamily of EIICs
have an elevator-type mechanism of transport, we need to visualize both
conformations in the same transporter to reveal the
conformational changes. To achieve this, Ren et al. 2018 first generated a structural
model of bcMalT in an inward-facing conformation by collective
variable-based steered molecular dynamics (CVSMD) simulation using the bcChbC structure as a guide.
During the simulation, the interface domain was kept static, and the
substrate-binding domain was steered toward the inward-facing
conformation. Since the substrate-binding domain moves relative to the
interface domain, distance changes between the two domains were expected.
Indeed, the CVSMD model showed that residues that were far away from each
other in the outward-facing structure became closer, for example,
residues T280 and D55 and residues N284 and E54. The pairs of residues predicted to become closer to
each other can be cross-linked by a mercury ion when mutated to
cysteine residues and thus provide an experimental validation to the
CVSMD model and the elevator-type mechanism of transport.
Ren et al. 2018 solved the crystal structure of bcMalT cross-linked in
an inward-facing conformation. The structure provided direct
experimental evidence that the substrate-binding domain can undergo a
rigid-body rotation toward the intracellular side. The structure also
showed conformational changes in other regions of the transporter that
accommodate the rigid-body movement of the substrate-binding domain.
|
This family belongs to the PTS-GFL Superfamily.
|
| References: |
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| Examples: |
| TC# | Name | Organismal Type | Example |
| 4.A.1.1.1 | Glucose porter (PtsG; GlcA; Umg) (transports D-glucose and α-methyl-D-glucopyranoside). The IIC domain has been crystallized, and x-ray data to 4.5 Å resolution have been described (Zurbriggen et al. 2010). A higher resolution structure appeared later (Ren et al. 2018). The system has been manipulated to engineer increased production of aromatic metabolites (Carmona et al. 2015, Vargas-Tah et al. 2015). The presence or absence of D-glucose reflects the transporter before and after release of the transported glucose into the cytoplasm. The transition associated with substrate release appears to require a subtle structural rearrangement in the region that includes hairpin 1 (Kalbermatter et al. 2017). Mlc (for makes large colonies) represses transcription of the genes
encoding enzyme I, HPr, EIIBCGlc and EIIABCDMan in defined media that lack PTS
substrates. When glucose is present, the unphosphorylated form of
EIIBCGlc sequesters Mlc to the cell membrane, preventing its
interaction with DNA (Plumbridge 2002, Joyet et al. 2013). The Vibrio Mlc functions similarly (Pickering et al. 2014). A small (43 aa) protein, SgrT, acts in tandem with a well-characterized small RNA during metabolic
stress, due to the accumulation of cytoplasmic sugar-Ps to help bacterial cells maintain balanced metabolism and continue
growing. SgrT acts on the glucose
transport system, inhibiting its activity under stress conditions in
order to allow cells to utilize alternative carbon sources (Lloyd et al. 2017). ptsG mRNA localization to the inner membrane, coupled with the membrane insertion of nascent peptide, mediates Hfq/SgrS-dependent ptsG mRNA destabilization, presumably by reducing second rounds of translation (Kawamoto et al. 2005). SgrT is a small protein of 43 aas that allosterically inhibits IICBGlc. while SgrS is a small RNA coompementary to ptsG mRNA that influences its expression. The sgrST operon is regulated by SgrR, a glucose-6-P-dependent transcriptional activator (Jeckelmann and Erni 2020).
| Bacteria, proteobacteria | PtsG/Crr; Glucose IICB/IIA complex of E. coli |
| |
| 4.A.1.1.10 | The α-glucoside-specific IICB, AglB (transports glucose, methyl-α-glucoside, maltitol, isomaltose, trehalulose α(1→1), turanose α(1→3), maltulose α(1→4), leucrose α(1→5), and palatinose α(1→6), but not sucrose (most resembles 4.A.1.1.4 and 4.A.1.1.8) (Pikis et al., 2006) | Bacteria | Glucose/α-glucoside IICB complex of Klebsiella pneumoniae (Q9AGA7) |
| |
| 4.A.1.1.11 | The glucose/maltose/maltotriose porter, MalT or PtsG (31% identical to 4.A.1.1.9) (Webb et al., 2007). Lactoperoxidase (LPO) shows promise in the prevention of dental caries. It has antimicrobial properties and is part of the non-specific antimicrobial immune system. A thiocyanate-iodide mixture strongly inhibited glucose and sucrose consumption as well as transmembrane PTS glucose transport. Thus, the LPO-iodide system had a strong inhibitory effect on biofilm growth and lactate synthesis (complete inhibition) (Magacz et al. 2023). | Bacteria | MalT/PtsG IICBA of Streptococcus mutans (Q8DS05) |
| |
| 4.A.1.1.12 | Maltose/Maltotriose PTS transporter, MalT (Shelburne et al., 2008) 631aas (68% identical to 4.A.1.1.11 from S. mutans | Bacteria | MalT IICBA of Streptococcus pyogenes (Q48WG5) |
| |
| 4.A.1.1.13 | Glucose porter, GlcA (IICBA). Glucose uptake is inhibited by 2-deoxyglucose and methyl-β-D-glucoside (Christiansen and Hengstenberg, 1999).
| Bacteria | GlcA of Staphylococcus carnosus (Q57071) |
| |
| 4.A.1.1.14 | Glucose porter GlcB (IICBA). Glucose uptake is inhibited by methyl-α-D-glucoside, methyl-β-D-glucoside, p-nitrophenyl-α-D-glucoside, o-nitrophenyl-β-D-glucoside and salicin, but not by 2-deoxyglucose. Mannose and N-acetylglucosamine are not transported (Christiansen and Hengstenberg, 1999). | Bacteria | GlcB of Staphylococcus carnosus (Q53922) |
| |
| 4.A.1.1.15 | N-acetyl glucosamine-specific PTS permease, GlcNAc IIBC/GlcNAc I-HPr-IIA (Johnson et al. 2008) | Proteobacteria | GlcNAc IIBC/GlcNAc I-HPr-IIA of Pseudomonas aeruginosa
GlcNAc IIBC (Q9HXN4)
GlcNAc I-HPr-IIA (Q9HXN5) |
| |
| 4.A.1.1.16 | PTS uptake porter for sucrose isomers, IICB (Thompson and Pikis 2012). | Fusobacteria | IICB for sucose isomers of Leptotrichia buccalis |
| |
| 4.A.1.1.17 | The Maltose group translocator, MalT of 470 aas and 10 TMSs. Takes up extracellular maltose, releasing maltose-phosphate into the cytoplasm. The 3-d structure at 2.55 Å resolution has been solved (McCoy et al. 2016; Vastermark and Saier 2016). | | MalT of Bacillus cereus |
| |
| 4.A.1.1.18 | Glucose-specific Enzyme IIBC of the PTS, PtsG. Essential for infectivity and virulence in mice although no other PTS Enzyme II is required (Khajanchi et al. 2016). | | IIBCGlc (PtsG) of Borrelia burgdorferi |
| |
| 4.A.1.1.19 | PTS α-glucoside transporter, subunit IICB (Francl et al. 2010). | | IICB of Lactobacillus gasseri |
| |
| 4.A.1.1.2 | N-Acetyl glucosamine (NAG) porter (NagE) (Plumbridge 2015). | Proteobacteria | NagE, the IICBANag complex of E. coli |
| |
| 4.A.1.1.20 | The N-acetylglucosamine PTS transporter/kinase, NagE2 (416 aas; IIC)/NagF (77 aas; IIB). The IIA specific for glucose (Crr) is the IIA for this system, and activity depends on Enzyme I and HPr (Nothaft et al. 2010). The genes encoding these enzymes are regulated by two transcription factors, DasR and AtrA, and the system serves as a sensor as well as a transporter/kinase (Nothaft et al. 2010). | | NagE2F of Streptomyces coelicolor |
| |
| 4.A.1.1.21 | Enzyme IIA of 168 aas. It is of the glucose type and can phosphorylate maltose via MalP (TC# 4.A..1.1.8), N-acetyl glucosamine via NagP (TC#4.A.1.1.7), sucrose via SacX (TC#4.A.1.2.10) and SacP (TC# 4.A.1.2.9), and trehalose via TreP (TC# 4.A.1.2.8), none of which have their own IIA protein (Morabbi Heravi and Altenbuchner 2018). Other Enzymes IIA could also phosphorlyate these sugars via their respective IIBC proteins. | | PtsA (YpqE) IIA protein of Bacillus subtilis |
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| 4.A.1.1.3 | Maltose/glucose porter (MalX) of 510 aas and 10 TMSs. This system functions with the glucose IIA protein (Reidl and Boos 1991; Jeckelmann and Erni 2020). | Bacteria | Maltose IICB complex (MalX) of E. coli |
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| 4.A.1.1.4 | The α-glucoside-specific IICB (MalB) (substrates probably include glucosyl-α-fructose disaccharides: trehalurose (α-1,1), turanose (α-1,3), malturose (α-1,4), leucrose (α-1,5) and palatinose (α-1,6)). | Bacteria | The α-glucoside IICB, MalB of Fusobacterium mortiferum |
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| 4.A.1.1.5 | N-Acetylglucosamine (NAG) porter (PtsBC1C2)(also may facilitate xylose transport) (Saito and Schrempf 2004). | Bacteria | The NAG IICC'B complex of Streptomyces olivaceoviridis (IIA not identified)
IICNag (PtsC1)
IIC'Nag (PtsC2)
IIBNag (PtsB) |
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| 4.A.1.1.6 | The glucosamine IICBA porter (GamP) (40% identical to 4.A.1.1.2) (Plumbridge 2015). The IIA domain in this protein can transfer the phosphoryl moiety to the maltose, N-acetylglucosamine, sucrose and trehalose PTS systems (MalP, NagP, SacP and TreP, respectively) (Morabbi Heravi and Altenbuchner 2018). | Bacteria; firmicute | GamP of Bacillus subtilis (gi2632521) |
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| 4.A.1.1.7 | The N-acetylglucosamine IICB porter (NagP; YflF) (45% identical to 4.A.1.1.2) (Plumbridge 2015). | Bacteria; firmicute | NagP of Bacillus subtilis (gi2443228) |
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| 4.A.1.1.8 | The maltose IICB porter (MalP; GlvC) (56% identical to 4.A.1.1.4) (Yamamoto et al., 2001). | Bacteria | MalP of Bacillus subtilis (IICBA) (P54715) |
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| 4.A.1.1.9 | The glucose IICBA porter (PtsG) 44% identical to 4.A.1.1.1) | Bacteria | PtsG of Bacillus subtilis (P20166) |
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| Examples: |
| TC# | Name | Organismal Type | Example |
| 4.A.1.2.1 | Sucrose porter (ScrA) | Bacteria | Sucrose IIBC complex (ScrA) of plasmid pUR400 from Salmonella typhimurium |
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| 4.A.1.2.10 | Sucrose porter and regulatory sensor, IIBC (SacX) (43% identical to 4.A.1.2.1) (Tortosa and Le Coq 1995). The IIA domains of PtsA, GamP, PtsG and GmuA can all phosphorylate the IIB domain in the SacX sensor (Morabbi Heravi and Altenbuchner 2018). | Bacteria | SacX of Bacillus subtilis (P15400) |
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| 4.A.1.2.11 | Aryl β-glucoside porter, IIBCA (BglP; SytA) (35% identical to 4.A.1.2.2) | Bacteria | BglP of Bacillus subtilis (P40739) |
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| 4.A.1.2.12 | The sucrose porter, PtsS (regulated by SugR which also regulates other enzymes II) (Engels and Wendisch, 2007) | Bacteria | PtsS (IIBCA) complex of Corynebacterium glutamicum (Q8NMD6) |
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| 4.A.1.2.13 | Trehalose PTS permease IIBC of 494 aas (Ells and Truelstrup Hansen 2011). | Firmicutes | Trehalose IIBC of Listeria monocytogenese |
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| 4.A.1.2.14 | PTS beta-glucoside transporter, EIIBCA of 672 aas and 12 predicted TMSs (Francl et al. 2010). | | EIIBCA of Lactobacillus gasseri |
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| 4.A.1.2.15 | PTS beta-glucoside transporter, EIIBCA of 624 aas and 10 predicted TMSs (Francl et al. 2010). | | EIIBCA of Lactobacillus gasseri |
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| 4.A.1.2.16 | PTS beta-glucoside transporter, EIIBCA of 647 aas and 10 predicted TMSs (Francl et al. 2010). | | EIIBCA of Lactobacillus gasseri |
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| 4.A.1.2.17 | N-acetylmuramic acid (MurNAc)-selective PTS transport system, MurP, IIBC, of 455 aas and 10 - 12 TMSs. | | MurP of Bacillus velezensis |
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| 4.A.1.2.18 | Glucose/fructose/glucosamine/mannose PTS IIABC system, FruA. FruA phosphorylates the sugar subrates on the 6-hydroxyl group (Mazé et al. 2007). | | FruA of Bifidobacterium breve |
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| 4.A.1.2.19 | Fructooligosaccharide uptake porter of 651 aas and 10 TMSs, Pts1BCA (Chen et al. 2015). | | Pts1CA of Lactobacillus plantarum |
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| 4.A.1.2.2 | β-Glucoside (salicin, arbutin, cellobiose, etc) group translocator, BglF. The bgl operon, encoding BglF, is regulated by antitermination when the RNA antiterminator protein, BglG, binds to one or both RAT sites in the mRNA (Gordon et al. 2015). | Bacteria; proteobacteria | BglF (IIBCAbgl) complex of E. coli |
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| 4.A.1.2.3 | β-Glucoside [arbutin-salicin-cellobiose] (ASC) group translocator, AscF (Desai et al. 2010). | Bacteria; proteobacteria | AscF (IICBAsc complex) of E. coli |
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| 4.A.1.2.4 | Trehalose porter, TreB (IIBC) which can take up maltose by facilitated diffusion (Decker et al. 1999). Can also transport sucrose at a low rate (Steen et al. 2014). | Bacteria | Trehalose IIBC complex of E. coli |
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| 4.A.1.2.5 | β-glucoside (methyl-β-glucoside, salicin, arbutin) porter, BglF [a V317A or V317M mutation allows it to transport cellobiose as well] (Kotrba et al., 2003) | Bacteria | β-glucoside IIBCA (BglF) of Corynebacterium glutamicum |
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| 4.A.1.2.6 | β-glucoside (Aesculin/arbutin) porter, BglP (Cote et al., 2000; Cote and Honeyman, 2003) | Bacteria | β-glucoside IIBCA (BglP) of Streptococcus mutans (AAF89975) |
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| 4.A.1.2.7 | N-Acetylmuramic acid porter, MurP (YfeV) (Dahl et al., 2004) | Bacteria | N-Acetylmuramate IIBC (MurP or YfeV) of E. coli (P77272) |
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| 4.A.1.2.8 | Trehalose porter, IIBC (TreP) (38% identical to 4.A.1.2.4) (Schöck and Dahl 1996; Ujiie et al. 2009). | Bacteria | TreP of Bacillus subtilis (P39794) |
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| 4.A.1.2.9 | Sucrose porter, IIBC (SacP) (55% identical to 4.A.1.2.1) | Bacteria | SacP of Bacillus subtilis (P05306) |
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