4.A.2 The PTS Fructose-Mannitol (Fru) Family

The Fru family is a large and complex family which includes several sequenced fructose, mannose and mannitol-specific porters as well as several putative PTS porters of unknown specificities. The fructose porters of this family phosphorylate fructose on the 1-position. Those of family 4.6 phosphorylate fructose on the 6-position. As is true of other members of the PTS-GFL superfamily, the IIC domains of these permeases probably have a uniform 10 TMS topology (Vastermark and Saier 2016; McCoy et al. 2016; Cao et al. 2011).

The IIA, IIB and IIC domains of the fructose- and mannitol-specific porters are demonstrably homologous. The IIB and IIC domains of the fructose porters are only distantly related to the corresponding domains of the mannitol porters. The IIB and IIC domains of these porters are homologous to those of the Glc family (TC #4.A.1) (Chang et al., 2004). However, the structure of the IIA domain of the mannitol porter of E. coli has been determined, and it proved to possess an α2β2α3 secondary structure, a structure which is very different from the β-sandwich structure of IIAGlc. Further, the IIC domains of the mannitol and fructose porters are almost as dissimilar from each other as they are from the glucose (TC #4.A.1) or lactose (TC #4.A.3) families.

This family belongs to the PTS-GFL Superfamily.



Araki N., Suzuki T., Miyauchi K., Kasai D., Masai E. and Fukuda M. (201). Identification and characterization of uptake systems for glucose and fructose in Rhodococcus jostii RHA1. J Mol Microbiol Biotechnol. 20(3):125-36.

Benchabane, H., L.A. Lortie, N.D. Buckley, L. Trahan, and M. Frenette. (2002). Inactivation of the Streptococcus mutans fxpC gene confers resistance to xylitol, a caries-preventive natural carbohydrate sweetener. J. Dent. Res. 81: 380-386.

Cai, L., S. Cai, D. Zhao, J. Wu, L. Wang, X. Liu, M. Li, J. Hou, J. Zhou, J. Liu, J. Han, and H. Xiang. (2014). Analysis of the transcriptional regulator GlpR, promoter elements, and posttranscriptional processing involved in fructose-induced activation of the phosphoenolpyruvate-dependent sugar phosphotransferase system in Haloferax mediterranei. Appl. Environ. Microbiol. 80: 1430-1440.

Cao, Y., X. Jin, E.J. Levin, H. Huang, Y. Zong, M. Quick, J. Weng, Y. Pan, J. Love, M. Punta, B. Rost, W.A. Hendrickson, J.A. Javitch, K.R. Rajashankar, and M. Zhou. (2011). Crystal structure of a phosphorylation-coupled saccharide transporter. Nature 473: 50-54.

Chang, A.B., R. Lin, W.K. Studley, C.V. Tran, and M.H. Saier, Jr. (2004). Phylogeny as a guide to structure and function of membrane transport proteins. Mol. Membrane Biol. 21: 171-181.

Comas, I., F. González-Candelas, and M. Zúñiga. (2008). Unraveling the evolutionary history of the phosphoryl-transfer chain of the phosphoenolpyruvate:phosphotransferase system through phylogenetic analyses and genome context. BMC Evol Biol 8: 147.

Delobbe, A., H. Chalumeau, and P. Gay. (1975). Existence of two alternative pathways for fructose and sorbitol metabolism in Bacillus subtilis Marburg. Eur J Biochem 51: 503-510.

Gaurivaud, P., F. Laigret, E. Verdin, M. Garnier, and J.M. Bové. (2000). Fructose operon mutants of Spiroplasma citri. Microbiology 146(Pt9): 2229-2236.

Gay, P. and A. Delobbe. (1977). Fructose transport in Bacillus subtilis. Eur J Biochem 79: 363-373.

Heravi, K.M. and J. Altenbuchner. (2014). Regulation of the Bacillus subtilis mannitol utilization genes: promoter structure and transcriptional activation by the wild-type regulator (MtlR) and its mutants. Microbiology 160: 91-101.

Jacobson, G.R., C.A. Lee, J.E. Leonard, and M.H. Saier, Jr. (1983). Mannitol-specific enzyme II of the bacterial phosphotransferase system. I. Properties of the purified permease. J. Biol. Chem. 258: 10748-10756.

Jacobson, G.R., L.E. Tanney, D.M. Kelly, K.B. Palman, and S.B. Corn. (1983). Substrate and phospholipid specificity of the purified mannitol permease of Escherichia coli. J. Cell. Biochem. 23: 231-240.

Johnson, D.A., S.G. Tetu, K. Phillippy, J. Chen, Q. Ren, and I.T. Paulsen. (2008). High-throughput phenotypic characterization of Pseudomonas aeruginosa membrane transport genes. PLoS Genet 4: e1000211.

Joyet, P., M. Derkaoui, H. Bouraoui, and J. Deutscher. (2015). PTS-Mediated Regulation of the Transcription Activator MtlR from Different Species: Surprising Differences despite Strong Sequence Conservation. J. Mol. Microbiol. Biotechnol. 25: 94-105.

Kroon, G.J.A., J. Grötzinger, K. Dijkstra, R.M. Scheek and G.T. Robillard (1993). Backbone assignments and secondary structure of the Escherichia coli enzyme-II mannitol A domain determined by heteronuclear three-dimensional NMR spectroscopy. Prot. Sci. 2: 1331-1341.

Lee, C.A. and M.H. Saier, Jr. (1983). Mannitol-specific enzyme II of the bacterial phosphotransferase system. III. The nucleotide sequence of the permease gene. J. Biol. Chem. 258: 10761-10767.

Lee, H.Y., M. Magotra, T.Y. Wong, C. Chakraborty, and J.K. Liu. (2012). ATP-dependent fructose uptake system in Deinococcus radiodurans. Appl. Microbiol. Biotechnol. 93: 1241-1248.

Leonard, J.E. and M.H. Saier, Jr. (1983). Mannitol-specific enzyme II of the bacterial phosphotransferase system. II. Reconstitution of vectorial transphosphorylation in phospholipid vesicles. J. Biol. Chem. 258: 10757-10760.

Manayan, R., G. Tenn, H.B. Yee, J.D. Desai, M. Yamada, and M.H. Saier, Jr. (1988). Genetic analyses of the mannitol permease of Escherichia coli: isolation and characterization of a transport-deficient mutant which retains phosphorylation activity. J. Bacteriol. 170: 1290-1296.

McCoy, J.G., Z. Ren, V. Stanevich, J. Lee, S. Mitra, E.J. Levin, S. Poget, M. Quick, W. Im, and M. Zhou. (2016). The Structure of a Sugar Transporter of the Glucose EIIC Superfamily Provides Insight into the Elevator Mechanism of Membrane Transport. Structure 24: 956-964.

Nguyen, T.X., M.R. Yen, R.D. Barabote, and M.H. Saier, Jr. (2006). Topological predictions for integral membrane permeases of the phosphoenolpyruvate:sugar phosphotransferase system. J. Mol. Microbiol. Biotechnol. 11: 345-360.

Patron K., Gilot P., Camiade E. and Mereghetti L. (2015). An homolog of the Frz Phosphoenolpyruvate:carbohydrate phosphoTransferase System of extraintestinal pathogenic Escherichia coli is encoded on a genomic island in specific lineages of Streptococcus agalactiae. Infect Genet Evol. 32:44-50.

Patron, K., P. Gilot, V. Rong, A. Hiron, L. Mereghetti, and E. Camiade. (2017). Inductors and regulatory properties of the genomic island-associated fru2 metabolic operon of Streptococcus agalactiae. Mol. Microbiol. 103: 678-697.

Pickl A., Johnsen U. and Schonheit P. (2012). Fructose degradation in the haloarchaeon Haloferax volcanii involves a bacterial type phosphoenolpyruvate-dependent phosphotransferase system, fructose-1-phosphate kinase, and class II fructose-1,6-bisphosphate aldolase. J Bacteriol. 194(12):3088-97.

Powell, B.S., D.L. Court, T. Inada, Y. Nakamura, V. Michotey, X. Cui, A. Reizer, M.H. Saier, Jr. and J. Reizer (1994). Novel proteins of the phosphotransferase system encoded within the rpoN operon of Escherichia coli. J. Biol. Chem 270: 4822-4839.

Reizer, J., A. Reizer, and M.H. Saier, Jr. (1995). Novel phosphotransferase system genes revealed by bacterial genome analysis--a gene cluster encoding a unique Enzyme I and the proteins of a fructose-like permease system. Microbiology 141(Pt4): 961-971.

Reizer, J., I.T. Paulsen, A. Reizer, F. Titgemeyer and M.H. Saier, Jr. (1996). Novel phosphotransferase system genes revealed by bacterial genome analysis: the complete complement of pts genes in Mycoplasma genitalium. Microbial Comp. Genomics. 1: 151-164.

Reizer, J., S. Bachem, A. Reizer, M. Arnaud, M.H. Saier, Jr., and J. Stülke. (1999). Novel phosphotransferase system genes revealed by genome analysis – the complete complement of PTS proteins encoded within the genome of Bacillus subtilis. Microbiology 145: 3419-3429.

Reizer, J., V. Michotey, A. Reizer, and M.H. Saier, Jr. (1994). Novel phosphotransferase system genes revealed by bacterial genome analysis: unique, putative fructose- and glucoside-specific systems. Protein. Sci. 3: 440-450.

Richards, V.P., P. Lang, P.D. Bitar, T. Lefébure, Y.H. Schukken, R.N. Zadoks, and M.J. Stanhope. (2011). Comparative genomics and the role of lateral gene transfer in the evolution of bovine adapted Streptococcus agalactiae. Infect Genet Evol 11: 1263-1275.

Rouquet, G., G. Porcheron, C. Barra, M. Répérant, N.K. Chanteloup, C. Schouler, and P. Gilot. (2009). A metabolic operon in extraintestinal pathogenic Escherichia coli promotes fitness under stressful conditions and invasion of eukaryotic cells. J. Bacteriol. 191: 4427-4440.

Sampaio, M.M., F. Chevance, R. Dippel, T. Eppler, A. Schlegel, W. Boos, Y.J. Lu, and C.O. Rock. (2004). Phosphotransferase-mediated transport of the osmolyte 2-O-α-mannosyl-D-glycerate in Escherichia coli occurs by the product of the mngA (hrsA) gene and is regulated by the mngR (farR) gene product acting as repressor. J. Biol. Chem. 279: 5537-5548.

Shakeri-Garakani, A., A. Brinkkötter, K. Schmid, S. Turgut, and J.W. Lengeler. (2004). The genes and enzymes for the catabolism of galactitol, D-tagatose, and related carbohydrates in Klebsiella oxytoca M5a1 and other enteric bacteria display convergent evolution. Mol. Genet. Genomics 271: 717-728.

Sugiyama, J.E., S. Mahmoodian and G.R. Jacobson (1991). Membrane topology analysis of Escherichia coli mannitol permease by using a nested-deletion method to create mtlA phoA fusions. Proc. Natl. Acad. Sci. USA 88: 9603-9607.

Suh, J.Y., J. Iwahara, and G.M. Clore. (2007). Intramolecular domain-domain association/dissociation and phosphoryl transfer in the mannitol transporter of Escherichia coli are not coupled. Proc. Natl. Acad. Sci. USA 104: 3153-3158.

Sun, T. and J. Altenbuchner. (2010). Characterization of a mannose utilization system in Bacillus subtilis. J. Bacteriol. 192: 2128-2139.

Tanzer, J.M., A. Thompson, Z.T. Wen, and R.A. Burne. (2006). Streptococcus mutans: fructose transport, xylitol resistance, and virulence. J. Dent. Res. 85: 369-373.

Van der Heiden, E., M. Delmarcelle, P. Simon, M. Counson, M. Galleni, D.I. Freedberg, J. Thompson, B. Joris, and M.D. Battistel. (2015). Synthesis and Physicochemical Characterization of D-Tagatose-1-Phosphate: The Substrate of the Tagatose-1-Phosphate Kinase in the Phosphotransferase System-Mediated D-Tagatose Catabolic Pathway of Bacillus licheniformis. J. Mol. Microbiol. Biotechnol. 25: 106-119.

Vastermark, A. and M.H. Saier, Jr. (2016). Time to Stop Holding the Elevator: A New Piece of the Transport Protein Mechanism Puzzle. Structure 24: 845-846.

Wen, Z.T., C. Browngardt, and R.A. Burne. (2001). Characterization of two operons that encode components of fructose-specific enzyme II of the sugar:phosphotransferase system of Streptococcus mutans. FEMS Microbiol. Lett. 205: 337-342.


TC#NameOrganismal TypeExample
4.A.2.1.1Fructose porter (FruAB) (fructose-1-P forming) Bacteria Fructose IIB'BC-IIAMH complex of E. coli

The FrwABCD putative transporter of unknown function. FruA is a 3-domain multiphosphoryl transfer protein: EIAni-HPr-IIAFru (Reizer et al., 1995).


FrwABCD of E. coli
FrwA (IIA) (P32670)
FrwB (IIB) (P69816)
FrwC (IIC) (P32672)
FrwD (IIB) (P32676)


The FryABC putative transporter of unknown function.  FryA is a 3-domain multiphosphoryl transfer protein: EI-HPr-IIAFru (Reizer et al., 1995).


FryABC of E. coli
FryA (IIA) (P77439)
FryB (IIB) (P69808)
FryC (IIC) (P77579)


The mannitol/glucitol transporter, MtlA (IICBAMtl) (Kumar et al., 2011)


MtlA (IICBA) of Vibrio cholerae (Q9KKQ7)


The fructose-specific PTS Enzyme IIABC FruA (Araki et al., 2011).


FruA of Rhodococcus jostii (Q0S1N2)


Fructose Enzyme II complex (IIAFru - IIBFru - IICFru) (based on homology)


IIABCFru of Haloterrigena turkmenica


Fructose-specific PTS, PtfABC (functions with 8.A.7.1.4 and 8.A.8.1.4; Pickl et al., 2012).  The transcriptional regulation of the fructose PTS in the very similar organism, Haloferax mediterranei, has shown that GlpR is a transcriptional activator (Cai et al. 2014).


Fructose Enzyme II, PtfABC (IIABC) complex of Haloferax volcanii 
PtfA (IIA) (D4GYE1)
PtfB (IIB) (D4GYE4)
PtfC (IIC) (D4GYE5) 


Fructose-specific Enzyme IIABC (Gaurivaud et al. 2000).


Fructose IIABC of Spiroplasma citri


Fructose-specific PTS permease, FruIIBC/FruI-HPr-IIA (Johnson et al. 2008).


FruIIBC/FruI-HPr-IIA of Pseudomonas aeruginosa
FruIIBC (Q9HY57)
FruI-HPr-IIA (Q9HY55)


Fructose PTS Enzyme IIBC, FruA.  The gene encoding the IIA protein is an inactive pseudogene, and fructose appears to be phosphorylated by an ATP-dependent mechanism (Lee et al. 2012).


FruA of Deinococcus radiodurans


The tagatose-specific PTS transporter/kinase, TagIIA-TPr/TagIIB'BC (tagatose-1-P forming) (Shakeri-Garakani et al. 2004).  TagIIA-TPr is a fusion of a IIA domain fused N-terminal to an HPr domain.  TagIIB'BC has and inactive IIB' domain fused N-terminal to the active C-terminal IIBC domains.  This arrangement resembles that for the E. coli fructose Enzyme II complex.


Tag PTS of Klebsiella pneumoniae


Mannitol porter (MtlA) (mannitol-1-P forming), the mannitol IICBA complex.  The enzyme-transporter has been alterred genetically, sequenced, purified, reconstituted and characterized (Jacobson et al. 1983, Leonard and Saier 1983, Lee and Saier 1983, Manayan et al. 1988). Intramolecular phosphoryl transfer between the A and B domains of IIMtl is rate-limited by chemistry and not by the rate of formation or dissociation of a stereospecific complex in which the active sites are optimally apposed (Suh et al. 2007). Substrates, in addition to D-mannitol, include D-glucitol (D-sorbitol), D-2-amino-2-deoxymannitol, D-2-deoxymannitol and D-arabitol (D-arabinitol) (Jacobson et al. 1983).


Mannitol IICBA complex of E. coli


Chromosomal fructose Enzyme IIABC (Fru1) of 654 aas; in an operon with fructose-1-P kinase (Patron et al. 2015).


Fru1 of Streptococcus agalactiae


Putative fructose Enzyme II complex, Fru3; IIA (148 aas)/IIBC (464 aas) (Richards et al. 2011).


IIA/IIBC of Streptococcus agalactiae


D-allose/D-ribose transporting Enzyme II complex (Fru2; IIA/IIB/IIC) (Patron et al. 2017). This system is similar to Frz of E. coli (TC#4.A.2.1.9) which is involved in environmental sensing, host adaptation and virulence (Patron et al. 2015).  The regulatory mechanism has been studied (Patron et al. 2017).


D-Ribose and D-allose transporting Enzyme II complex of Streptococcus agalactiae
IIA, 149 aas
IIB, 103 aas
IIC, 367 aas


The tagatose-1-P-forming tagatose phosphorylating Enzyme IIA/IIBC, TagM/L (Van der Heiden et al. 2015). The product is phosphorylated by tagatose-1-P kinase (TagK), and then cleaved by tagatose-1,6-bisphosphate aldolase (GatY).


TagLK of Bacillus licheniformis
TagL (IIBCTag), 466 aas
TagM (IIATag), 152 aas


Cryptic mannitol permease, CmtA (IICB; 462 aas; 9 - 10 TMSs) - CmtB (IIA; 147 aas.)

CmtA-CmtB of E. coli


Fructose-like PTS Enzyme II complex, FrvA (IIA of 148 aas) - FrvB (IIBC of 483 aas and 9 predicted TMSs) (Reizer et al. 1994).

FrvIIa/IIBC of E. coli


Fructose-specific Enzyme I-HPr-Enzyme IIABC complex, all encoded within a single operon with genes in the order: ptsC (IIC), ptsA (IIA), ptsH (HPr), ptsI (Enzyme I) and ptsB (IIB) (Comas et al. 2008).

Fructose Enzyme II complex including EI and HPr of Haloarcula marismortui


Multicomponent PTS system including all of the proteins required for mannitol (or fructose) transport and phosophorylation, MtpMtl, with the domain order of MtlCBA-PtsHI. Specificity towards mannitol was inferred from the closest homology hits in TCDB.

MtpMtl of Desulfobacterium autotrophicum HRM2


The 2-O-α-mannosyl D-glycerate porter (2-O-α-mannosyl D-glycerate-6-P forming), MngA (HrsA) (Sampaio et al., 2004).  The phosphorylated product is hydrolyzed to manose-6-P and glycerate by MngB, an α-mannosidase.


2-O-α-mannosyl D-glycerate IIABC complex of E. coli


The fructose porter, FruA (fructose-1-P forming IIABC) (Delobbe et al. 1975) FruA is 39% identical to 4.A.2.1.1). fructose can be metabolized to Fru-1-P via this system as well as Fru-6-P by another PTS system (Gay and Delobbe 1977).


Fructose IIABC of Bacillus subtilis (gi2633811)


The mannitol porter (MtlA) (mannitol-1-P forming), MtlAF.  The system is encoded by the mannitol catabolic operon, mtlAFD, and is regulated by the transcription factor, MtlR (Joyet et al. 2015). MtlR contains an N-terminal helix-turn-helix motif followed by an Mga-like domain, two PTS regulatory domains (PRDs), an Enzyme IIBGat-like domain and an Enzyme IIAMtl-like domain, the last four of which can be phosphorylated by the PTS.  MtlR proteins are also found in Geobacillus stearothermophilus and Lactobacillus casei, but the mechanisms of their action are different (Joyet et al. 2015).  The dephosphorlyated form of the protein activates transcription (Heravi and Altenbuchner 2014).

Bacteria; firmicute

Mannitol IICB/A (MtlA/F) of Bacillus subtilis (P42956)


The mannose porter (ManP) (37% identical to 4.A.2.1.1). It is encoded in an operon with 3 genes:  manP-manA-yjdD, where manP codes for the IIBCA mannose transporter, manA codes for a mannose-6-P isomerase and YjdD  codes for a 5-formyltetrahydrofolate cyclo-ligase, characterized in B. anthrasis.  Expression of the operon is regulated by ManP and ManR, an activator, in response to external mannose (Sun and Altenbuchner 2010). 


Mannose IIBCA of Bacillus subtilis (gi2633555)

4.A.2.1.7The fructose inducible fructose/xylitol porter, FruI (Benchabane et al., 2002; Tanzer et al., 2006; Wen et al., 2001)BacteriaFruI (IIABC) of Streptococcus mutans (DAA01814)
4.A.2.1.8The constitutive fructose porter FruC/FruD (Benchabane et al., 2002; Tanzer et al., 2006; Wen et al., 2001)BacteriaFruC/D (IIBC/IIA) of Streptococcus mutans
FruC (IIBC) (AAN57895)
FruD (IIA) (DAA01808)

The FrzABC PTS putative transporter (promotes bacterial fitness under stress conditions and promotes fimbrial (fim) gene expression indirectly (Rouquet et al., 2009). Might transport D-tagatose, D-psicose and/or D-sorbose, or a disaccharide of these (Rouquet et al. 2009); involved in environmental sensing, host adaptation and virulence (Patron et al. 2015).


FrzABC of E. coli
FrzA (IIA) (Q1R4S9)
FrzB (IIB) (Q8FC73)
FrzC (IIC) (Q1R4T1)


TC#NameOrganismal TypeExample