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)
The L-ascorbate transporting and phosphorylating group translocator, SgaTBA or UlaCBA (SgaT = UlaA = YjfS; SgaB = UlaB = YjfT; SgaA = UlaC = PtxA = YjfU) (Hvorup et al., 2003; Zhang et al., 2003). Two conformations of the 3-d structure have been determined at 1.65 and 2.35 Å resolution, respectively (Luo et al., 2015). UlaA (SgaT) forms a homodimer with a novel fold. Each UlaA protomer consists of 11 TMSs arranged into a 'V-motif' domain and a 'core' domain. The V motifs form the interface between the two protomers, and the core-domain residues coordinate vitamin C. Alternating access of the substrate to the two sides of the cell membrane may be achieved through rigid-body rotation of the core relative to the V motif (Luo et al., 2015; Zhang et al., 2003). This structure does not resemble the ChbC structure (TC# 4.A.3.2.8).
L-ascorbate (L-Asc) IIC-IIB-IIA complex of E. coli
Virulence-associate PTS, VpeABC (substrate unknown). Essential for virulence and normal colonization of the kidney and intestine by uropathogneic E. coli (UPEC) (Martinez-Jéhanne et al. 2012).
VpeABC of E. coli AL511
Putative Enzyme IIC specific for ascorbate of 410 aas
IICasc of Clostridium carboxidivorans
Uncharacterized protein of 420 aas; possibly a IIC or IICB PTS protein (based on homology); SgaT/UlaA
UlaA/SgaT of Klebsiella pneumoniae
Uncharacterized protein of 424 aas and 12 TMSs
UP of Turicibacter sanguinis
The PTS ascorbate transporter subunits IIBC (596 aas and 11 TMSs) and IIA (155 aas).The 3-d structure has been determined at high resolution of the inward open configuration, showing that ascorbate translocation can be achieved by a rigid-body movement of the substrate-binding core domain relative to the V motif domain, which brings along the transmembrane helices TM2 and TM7 of the V motif domain to undergo a winding at the pivotal positions (Luo et al. 2018). This completes the picture of the transport cycle of the ascorbate superfamily of membrane-spanning EIIC components of the PTS.
Ascorbate transporter of Pasteurella multocida