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4.A.7 The PTS L-Ascorbate (L-Asc) Family

A single PTS permease of the L-Asc family of PTS permeases has been functionally characterized. This is the SgaTBA system (Tschieu et al., 2002), renamed UlaABC (utilization of L-ascorbate) by Yew and Gerlt (2002). The SgaTBA permease consists of three proteins: SgaT, SgaB, and SgaA. SgaT is a 12 TMS protein, possibly very distantly related to the MFS hexuronate permease of E. coli (TC #2.A.1.14.2). It presumably functions as a PTS IIC protein. This gene product as well as SgaB (homologous to but distantly related to IIB proteins of the PTS LAC family (TC #4.A.3)) and SgaA (homologous to but distantly related to IIA proteins of the PTS FRU family (TC #4.A.2)), are all essential for anaerobic L-ascorbate utilization, transport and phosphorylation (Zhang et al., 2003). This is the first documented example where the two sugar-specific energy-coupling proteins of a PTS permease are more closely related to the proteins of two different families. The sga regulation is controlled by the nearby YjfQ repressor (Campos et al., 2002; Zhang et al., 2003).

Homologues of SgaT, like other PTS protein homologues, have been identified in a large number of evolutionarily divergent bacteria but not in eukaryotes (Zhang et al., 2003). Bacteria which encode SgaT homologues include numerous Gram-negative proteobacteria as well as many low and high G+C Gram-positive bacteria. Except for species of Corynebacterium, Streptomyces and Bacillus, almost all organisms possessing SgaTBA homologues are human/animal pathogens. Several organisms have two or more SgaT paralogues including E. coli which has three. In E. coli, the SgaTBA homologues cannot transport L-ascorbate since the the sgaA, sgaB and sgaT mutants proved to be negative for L-ascorbate utilization, uptake and phosphorylation. In some of the homologues found in other bacteria, SgaB domains are fused C-terminal to the SgaT domains. For example, this is true of putative transporters in Vibrio cholerae (AAP96157; 586 aas), Pasteurella multocida (AAK02848; 625 aas) and Mycoplasma pulmonis (CAC13371; 650 aas). Homologues of SgaB and SgaA, but not SgaT, are also found in transcriptional activator proteins where they function in regulation rather than sugar transport (Greenberg et al., 2002).

The L-Asc Family is related to the Gat Family (TC# 4.A.5). The discovery of a nonhomologous, nontransporting Enzyme II complex specific for dihydroxyacetone resembling in sequence functionally characterized ATP-dependent dihydroxyacetone kinases (Gutknecht et al., 2001) illustrates the versatility of the PTS in recruiting proteins that evolved for other catalytic purposes into this PEP-dependent phosphotransferase system (Hvorup et al. 2003; Saier et al. 2005). Since not all established Enzyme II complexes are homologous (Saier and Reizer, 1994), the use of SgaT as an Enzyme IIC of the PTS, while representing a unique and novel example, does not establish a new principle. Nevertheless, the mechanism of phosphoryl transfer from SgaB-P to the substrate sugar acid may well prove to exhibit unique features.

The group translocation reaction catalyzed by SgaTBA is:

L-ascorbate (out) L-ascorbate-6-phosphate (in)

This family belongs to the: PTS-AG Superfamily.

References associated with 4.A.7 family:

Campos, E., J. Aguilar, L. Baldoma, and J. Badia. (2002). The gene yjfQ encodes the repressor of the yjfR-X regulon (ula), which is involved in L-ascorbate metabolism in Escherichia coli. J. Bacteriol. 184: 6065-6068. 12374842
Greenberg, D.B., J. Stülke, and M.H. Saier, Jr. (2002). Domain analysis of transcriptional regulators bearing PTS-regulatory domains. Res. Microbiol. 153: 519-526. 12437213
Gutknecht, R., R. Beutler, L.F. Garcia-Alles, U. Baumann, and B. Erni. (2001). The dihydroxyacetone kinase of Escherichia coli utilizes a phosphoprotein instead of ATP as phosphoryl donor. EMBO J. 20: 2480-2486. 11350937
Hvorup, R., A.B. Chang, and M.H. Saier, Jr. (2003). Bioinformatic analyses of the bacterial L-ascorbate phosphotransferase system permease family. J. Mol. Microbiol. Biotechnol. 6: 191-205. 15153772
Luo P., Yu X., Wang W., Fan S., Li X. and Wang J. (2015). Crystal structure of a phosphorylation-coupled vitamin C transporter. Nat Struct Mol Biol. 22(3):238-41. 25686089
Luo, P., S. Dai, J. Zeng, J. Duan, H. Shi, and J. Wang. (2018). Inward-facing conformation of l-ascorbate transporter suggests an elevator mechanism. Cell Discov 4: 35. 30038796
Martinez-Jéhanne, V., C. Pichon, L. du Merle, O. Poupel, N. Cayet, C. Bouchier, and C. Le Bouguénec. (2012). Role of the vpe carbohydrate permease in Escherichia coli urovirulence and fitness in vivo. Infect. Immun. 80: 2655-2666. 22615242
Saier, M.H., Jr. and J. Reizer. (1994). The bacterial phosphotransferase system: new frontiers 30 years later. Mol. Microbiol. 13: 755-764. 7815935
Saier, M.H., R.N. Hvorup, and R.D. Barabote. (2005). Evolution of the bacterial phosphotransferase system: from carriers and enzymes to group translocators. Biochem Soc Trans 33: 220-224. 15667312
Tchieu, J.H., V. Norris, J.S. Edwards, and M.H. Saier, Jr. (2002). The complete phosphotransferase system in Escherichia coli. In The Bacterial Phosphotransferase System (JMMB Symposium Series, Vol. 5), Chapter 2 (M.H. Saier, Jr., ed.). Wymondham, UK: Horizon Scientific Press, pp. 9-51. 11361063
Yew, W.S. and J.A. Gerlt. (2002). Utilization of L-ascorbate by Escherichia coli K-12: assignments of functions to products of the yif-sga and yia-sgb operons. J. Bacteriol. 184: 302-306. 11741871
Zhang, Z., M. Aboulwafa, M.H. Smith, and M.H. Saier, Jr. (2003). The ascorbate transporter of Escherichia coli. J. Bacteriol. 185: 2243-2250. 12644495