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. However, their distant relationships, the lack of homology of SgaT with any other PTS IIC, and our inability to establish homology of SgaT with any other protein in TCDB warrants inclusion in a distinct family. 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 archaea or 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 independent evolutionary origin of SgaT as compared with the other established PTS IIC proteins (families 4.A.1-6) is not surprising in view of the previously established independent origins of at least some of the latter. Moreover, the recent 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. 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)