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Tripartite dicarboxylate:H+ symporter (substrates include: fumarate, D- and L-malate, succinate, succinamide, orotate, iticonate and mesaconate) (Forward et al., 1997)
DctPQM dicarboxylate transporter of Rhodobacter capsulatus
DctP (R)
DctQ (M, 4 TMS)
DctM (M, 12 TMS)

The 2,3-diketo-L-gulonate (2,3-DKG) transporter, YiaMNO [2,3-KDG is a breakdown product of L-ascorbate] (Thomas et al., 2006)
YiaMNO of E. coli
YiaM (M, 4 TMSs; most like TauL) (P37674)
YiaN (M, 12 TMSs; most like DctM) (P37675)
YiaO (R, like DctP and TauK) (P37676)

Na+-dependent (smf-driven) sialic acid (N-acetyl neuraminic acid) transporter, SiaTP (Allen et al., 2005; Severi et al., 2005; Johnston et al., 2008). SiaT is also called SiaQM (Mulligan et al., 2009).  Also transports the related sialic acids, N-glycolylneuraminic acid (Neu5Gc) and 3-keto-3-deoxy-D-glycero-D-galactonononic acid (KDN) (Hopkins et al. 2013). Peter et al. 2024 have proposed that conformational coupling of the sialic acid TRAP transporter HiSiaQM with its substrate binding protein HiSiaP accounts for its energetic features. The SBP can adopt an open- or closed state depending on the presence of substrate. The two transmembrane domains of TRAP transporters form a monomeric elevator whose function is strictly dependent on the presence of a sodium ion gradient (Peter et al. 2024).  cryo-EM structure of the Haemophilus influenzae N-acetylneuraminate TRAP transporter (HiSiaQM) at 2.99 Å resolution (extending to 2.2 Å at the core), revealed new features (Currie et al. 2024).


SiaTP of Haemophilus influenzae
SiaT (a fusion protein equivalent to both DctM and DctQ) (616 aas; 16 TMSs) (P44543)
SiaP (R) (P44542)

Putative tripartite taurine uptake system, TauKLM (Bruggemann et al., 2004; Denger et al., 2006)

TauKLM of Rhodobacter sphaeroides
TauK (Rsph2615) (R) (Q3IVI6)
TauL (Rsph2614) (M, 4 TMS) (Q3IVI5)
TauM (Rsph2613) (M, 12 TMS) (Q3IVI4)

The putative rhamnogalacturonide transporter (Rodionov et al. 2004)

RhiABC of Salmonella typhimurium
RhiA (R) (P43020)
RhiB (M, 4 TMSs) (Q8ZKR9)
RhiC (M, 12 TMSs) (Q8ZKS0)

The Na+-dependent sialic acid uptake porter, SiaPQM. SiaQ and SiaM form a 1:1 stoichiometric complex (Mulligan et al., 2012).  The structure of a Vibrio ortholog has been determined by cryoEM (Peter et al. 2022). The protein complex is composed of 16 TMSs in SiaQ (4 TMSs) and SiaM (12 TMSs) that are structurally related to multimeric elevator-type transporters. The idiosyncratic Q-domain of TRAP transporters enables the formation of a monomeric elevator architecture.  A model of the tripartite PQM complex is experimentally validated and reveals the coupling of the substrate-binding P protein to the transporter domains. Peter et al. 2022 studied the formation of the tripartite complex and investigated the impact of interface mutants. The 3-D structure of the he cryo-EM structure of the sialic acid TRAP transporter SiaQM from Photobacterium profundum at 2.97 Å resolution. SiaM comprises a "transport" domain and a "scaffold" domain, with the transport domain consisting of helical hairpins as seen in the sodium ion-coupled elevator transporter VcINDY. The SiaQ protein forms intimate contacts with SiaM to extend the size of the scaffold domain, suggesting that TRAP transporters may operate as monomers, rather than the typically observed oligomers for elevator-type transporters. Davies et al. 2023 identified the Na+ and sialic acid binding sites in SiaM from Photobacterium profundum at 2.97 Å resolution and demonstrated a strict dependence on the substrate-binding protein SiaP for uptake. They reported the SiaP crystal structure that, together with docking studies, suggested the molecular basis for how sialic acid is delivered to the SiaQM transporter complex. They proposed a model for substrate transport by TRAP proteins as an 'elevator-with-an-operator' mechanism (Davies et al. 2023).

SiaPQM of Vibrio cholerae
SiaP (R) (Q9KR64)
SiaQ (M, 4TMSs) (B9TSN0)
SiaM (M, 12 TMSs) (B9TSM9)

The malonate uptake transporter, MatPQM. Regulated by the GtrA transcriptional activator (Chen et al. 2010). MatM is fused in a single protein C-terminal to MatA (malonyl-CoA decarboxylase).

MatPQM of Sinorhizobium meliloti
MatP (R) (Q930W1)
MatQ (M, 4TMSs) (Q930W2)
MatM (M, 12TMSs) (Q930W3)

Sialic acid uptake transporter, DctMPQ

DctMPQ of E. coli 
DctM (Q8FA80)
DctQ (Q8FA79)
DctP (Q8FA78) 

The possible disulfide 3,3'-dithiodipropionic acid (DTDP) tripartite transporter, DctMPQ (Wübbeler et al. 2014).  More probably takes up an array of oxidized sugar onic acids, D-gluconate, D-galactonate, L-arabonate, D-fuconate and D-xylonate. The sugars are oxidized by a broad-range, membrane bound sogar oxidase.  The acids that have been studied kineticall have Kms between 8 and 15 μM (Meinert et al. 2017; ).

DTDP transporter of Advenella mimigardefordensis strain DPN7
DctM (M, large)
DctP (R)
DctQ (M, small)

Transporter for lignin derived aromatic compounds, TarPQM (Salmon et al. 2013).  The purple photosynthetic bacterium Rhodopseudomonas palustris is able to grow photoheterotrophically under anaerobic conditions on a range of phenylpropeneoid lignin monomers, including coumarate, ferulate, caffeate, and cinnamate. TarPQM is encoded at the same locus as CouPSTW (TC# 3.A.1.4.11) and several other genes involved in coumarate metabolism. The periplasmic binding-protein of this system (TarP) binds coumarate, ferulate, caffeate, and cinnamate with nanomolar KD values. Thus, R. palustris uses two redundant but energetically distinct primary and secondary transporters that both employ high-affinity periplasmic binding-proteins to maximize the uptake of lignin-derived aromatic substrates from the environment (Salmon et al. 2013).

TarPQM of Rhodopseudomonas palustris
TarP (R; 336 aas)
TarQ (small M; 217 aas)
TarM (large M; 435 aas)

Tripartite high affinity ectoine/hydroxyectoine uptake system (Grammann et al., 2002). Deletion leads to increased rates of ectoine excretion (Hobmeier et al. 2022). In the absence of the substrate-binding protein, TeaA, an overexpression of both subunits TeaBC facilitated a three-fold increased excretion rate of ectoine export. Individually, the large subunit TeaC showed an approximately five times higher extracellular ectoine concentration per dry weight compared to TeaBC shortly after its expression was induced. This led to the possibility that only the large subunit, TeaC, is required for channel function (Hobmeier et al. 2022).

TeaABC ectoine transporter of Halomonas elongata
TeaA (R)
TeaB (M, 4 TMS)
TeaC (M, 12 TMS)

2-Oxoglutarate, 2OG (α-ketoglutarate, αKG) uptake porter of 677 aas and 20 TMSs (Large + small subunits fused) plus a periplasmic solute binding protein of 318 aas and 1 N-terminal TMS.

αKG uptake porter of Shewanella oneidensis

Putative transporter
Putative transporter of Fusobacterium nucleatum (gi 19704274)

DctM4Q4P4 three component TRAP-T transporter that may take up phenylacetate and phenylpyruvate (Dörries et al. 2016).

DctM4Q4P4 of Desulfococcus multivorans

Three component TRAP-T transporter, DctM9Q9P9; may take up phenylacetate and phenylalanine (Dörries et al. 2016).

TRAP-T uptake system of Desulfobacula toluolica Tol2
DctM9, 430 aas and 14 TMSs; 90% identical to DctM4 (TC# 2.A.56.1.14)
DctQ9, 160 aas and 4 TMSs; 75% identical to DctQ4 (TC# 2.A.56.1.14)
DctP9, 358 aas and 1 TMS; 79% identical to DctP4 (TC# 2.A.56.1.14)

Uncharacterized TRAP-T family with DctM, DctQ and DctP; may transport dicarboxylic acids (by similarity).

TRAP-T family system of Gammaproteobacteria bacterium (marine metagenome) DctM, 433 aas, 10 TMSs, MBI80045
DctQ, 168 aas and 4 TMSs, MBI80046
DctP,  377 aas and 1 N-terminal TMS, MBI80047

2-oxoglutarate transporter with two components, a 20 - 22 TMS integral membrane protein of 674 aas and a solute-binding receptor with 317 aas and one N-terminal TMS. The former protein is 81% identical to the large membrane protein with TC# 2.A.56.1.12, and the latter is 74% identical to the binding protein constituent of TC# 2.A.56.1.12. The former two membrane protein homologues seem to have the same topologies with 20 - 22 TMSs.

2-oxoglutarate uptake TRAP transporter of Pseudomonas stutzeri

TRAP transporter with one membrane constituent (743 aas and 22 TMSs) and one receptor (330 aas and 1 N-terminal TMS). May transport dicarboxyic acids: 2-oxoglutarate, fumarate, L-malate and succinate. The membrane constituent is 37% identical to that in TC# 2.A.56.1.17, and the receptor is 21% identical to the receptor in TC# 2.A.56.1.17.

TRAP transporter of Dinoroseobacter shibae

The putative outer membrane anion-selective porin, TsaT, of 338 aas and probably 1 N-terminal TMS (Mampel et al. 2004). Although it was reported to be an outer membrane porin, it is homologous to periplasmic binding receptors of the TRAP-T family. It previously had TC# 9.A.56.1.1.

TsaT of Comamonas testosteroni (Pseudomonas testosteroni) (Q8KR68)

TRAP transporter for a hydrophobic substrate (3-d structure known; tp0958 has 18-20 TMSs) (Deka et al., 2012). The substrate could be a lipoprotein, tp0956 (O83922) which is encoded in the same operon with tp0957 and tp058. This protein differs from all other members of the TRAP-T family in having 19 predicted TMSs with extra TMSs at its N-terminus.

TRAP-T transporter of Treponema pallidum
tp0957 (R) (O83923)
tp0958 (M) (O83924) 

Tripartite glutamate:Na+ symporter (Quintero et al., 2001)
GtrABC glutamate:Na+ symporter of Synechocystis strain PCC6803
GtrA (M) (like DctQ)
GtrB (M) (like DctM)
GtrC (R) (like GlnH of E. coli)

Tripartite 4-chlorobenzoate symporter (also binds and may transport 4-bromo-, 4-iodo-, and 4-fluorobenzoate and with a lower affinity, 3-chlorobenzoate, 2-chlorobenzoate, 4-hydroxybenzoate, 3-hydroxybenzoate, and benzoate) (Chae and Zylstra, 2006)
FcbT1/T2/T3 of Comamonas sp. strain DJ-12
FcbT1 (R) (AAF16407)
FcbT2 (M-sm) (AAF16408)
FcbT3 (M-lg) (AAF16409)

The 2-oxo monocarboxylate transporter (Pernil et al., 2010). Transports pyruvate which is inhibited by various 2-ketoacids.

The 2-oxo monocarboxylate transporter of Anabaena (nostoc) sp. strain PCC7120
DctQ (Alr3026) (Q8YSQ8)
DctM (Alr3027) (Q8YSQ7)
DctP (Alr3028) (Q8YSQ6) 

The 2-ketomonocarboxylate transporter (presented in order of affinity - 2-oxovalerate [highest affinity, KD=0.1 μM], 2-oxoisovalerate, 2-oxobutyrate, 2-oxoisocaproate, 2-oxo-3-methylvalerate, pyruvate [lowest affinity, KD=3 μM]) (Thomas et al., 2006).

The 2-ketomonocarboxylate transporter of Rhodobacter capsulatus
DctM-2, M-large (D5ATK1)
DctQ-2, M-small (D5ATK0)
DctP-2, Receptor (R) (D5ALT6)