2.A.42 The Hydroxy/Aromatic Amino Acid Permease (HAAAP) Family

The HAAAP family includes three well-characterized aromatic amino acid:H+ symport permeases of E. coli: a high affinity tryptophan-specific permease, Mtr, a low affinity tryptophan permease, TnaB, and a tyrosine-specific permease, TyrP, as well as two well-characterized hydroxy amino acid permeases, the serine permease, SdaC, of E. coli, and the threonine permease, TdcC, of E. coli. It also includes a cysteine uptake porter, CyuP (YhaO). These proteins possess 403-443 amino acyl residues and exhibit eleven putative or established TMSs. They all function in amino acid uptake. Homologues are present in a large number of Gram-negative and Gram-positive bacteria. These proteins exhibit topological features common to the eukaryotic amino acid/auxin permease (AAAP) family (TC #2.A.18), and they exhibit limited sequence similarity with some of them. Since members of the HAAAP family exhibit limited sequence similarity with the large APC family (TC #2.A.3), all of these proteins may be related.

SdaC of E. coli (TC #2.A.42.2.1) is also called DcrA, and together with a periplasmic protein DcrB (P37620), it has been reported to play a role in phage DNA uptake in conjunction with an outer membrane receptor of the OMR family (TC #1.B.14). Thus, FhuA (TC #1.B.14.1.4) transports phage T5 DNA while BtuB (TC #1.B.14.3.1) transports phage C1 DNA (Samsonov et al., 2002). DcuB is a putative lipoprotein found only in enteric bacteria.

The generalized transport reaction catalyzed by proteins of the HAAAP family is:

Amino acid (out) + nH+ (out) → Amino acid (in) + nH+ (in).


This family belongs to the APC Superfamily.



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TC#NameOrganismal TypeExample
2.A.42.1.1Tyrosine permease Bacteria TyrP of E. coli (P0AAD4)

High affinity (3 μM tryptophan permease, Mtr.  Also transports indole. It functions to scavenge trace amounts of tryptophan in the medium for protein synthesis when concentrations are very low (Gu et al. 2013).


Mtr of E. coli (P0AAD2)


Low affinity (70 μM) tryptophan permease, coregulated with the enzyme tryptophanase.  It  functions in tryptohan degradation, yielding carbon and nitrogen (Gu et al. 2013).


TnaB of E. coli

2.A.42.1.4Tyrosine permease (most similar in sequence to Mtr of E. coli)BacteriaTutB of Erwinia herbicola

Putative aromatic amino acid permease


Putative ArAA permease of Francisella tularensis


γ-aminobutyric acid (GABA):Na+ symporter (Zhao et al. 2012).  The GABA Km is 40 µM, and uptake is inhibited by L-Asn and L-Gln.


GabP of Corynebacterium glutamicum


Aromatic amino acid permease of 367 aas and 11 TMSs

ArAAAP family member of Thermococcus barophilus


Uncharacterized amino acid uptake porter of 399 aas and 11 TMSs.

UP of Candidatus Wolfebacteria bacterium


TC#NameOrganismal TypeExample
2.A.42.2.1Serine permease Bacteria SdaC of E. coli (P0AAD6)
2.A.42.2.2Threonine/Serine permease Bacteria TdcC of E. coli (P0AAD8)

Inner membrane transport protein, YhjV, of 423 aas and 11 TMSs. It may play a role in the transport of fluorophores (fluorescent dyes) (Salcedo-Sora et al. 2021) as well as melatonin (Yang et al. 2022).


YhjV of Escherichia coli


Inner membrane inducible, anaerobic, cysteine uptake transport protein, CyuP (DlsT; YhaO) of 443 aas and 11 TMSs (Loddeke et al. 2017). It is in a bicistronic operon with CyuA, an iron-sulfur-containing cysteine desulfidase, and this operon is regulated by the CyuR protein and induced maximally under anaerobic conditions. L-cysteine, D-cysteine, and a few other sulfur-containing compounds can serve as inducers. This system has been characterized both in E. coli and in S. enterica (Loddeke et al. 2017).


CyuP of Escherichia coli

2.A.42.2.5Inner membrane transport protein YqeGBacteria

YqeG of Escherichia coli