The 4-Toluene Sulfonate Uptake Permease (TSUP) family (TC# 2.A.102) is also referred to as the TauE/SafE/YfcA/DUF81 Family. Although its members have not been rigorously characterized, evidence is available that at least some members function in the transport of sulfur containing organic compounds. These include 4-toluene sulfonate which may be transported by the TsaS of Cupriavidus necator (TC# 2.A.102.1.1), sulfolactate which may be exported by the TauE protein of Cupriavidus necator (TC# 2.A.102.2.1) and sulfoacetate which may be exported by the SafE1 protein of Neptuniibacter caesariensis (TC# 2.A.102.2.2). Another member of the TSUP family, TsaS of Comamonas testosteroni, has been reported to function in the uptake of 4-toluene sulfonate. None of these functional assignments can be considered to be certain.
Contents
The TSUP family has been characterized using bioinformatic approaches. Shlykov et al. (2012) showed that prokaryotic members of the family outnumber the eukaryotic members substantially, and in prokaryotes, but not eukaryotes, extensive lateral gene transfer occurred during family evolution. Despite unequal representation, homologues from the three taxonomic domains of life share well-conserved motifs. A prototypical eight TMS topology apparently arose by intragenic duplication of a four transmembrane segment (TMS) unit, and possibly, a two TMS α-helical hairpin structure was the precursor of the 4 TMS repeat unit. This may be true of the entire TOG superfamily. Genome context analyses confirmed the proposal of a sulfur-based compound transport role for many TSUP homologues, but functional outliers appear to be prevalent as well. The TSUP family is an established member of the Transporter/Opsin/G protein-coupled receptor (TOG) superfamily.
Function
Due to the great sequence divergence among homologues, functions for most TSUP family members have not been assigned and cannot be assumed. In fact, the few analyses that have been performed with TSUP homologues suggest differing functionalities. However, potential substrates including several sulfur-based compounds have been proposed through bioinformatics.
Structure
Members of the TSUP family express a protypical 8 transmembrane segment (TMS) topology which arose from an intragenic duplication of a 4 TMS unit. Possibly, a two TMS α-helical hairpin structure was the precursor of the 4 TMS repeat unit. Related proteins likely arose via the same pathway: 2→4→8 TMSs followed by loss of a TMS either at the N- or C-terminus, depending on the family, to give the more frequent 7 TMS topology.