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).