1.A.29 The Urea/Amide Channel (UAC) Family
Proteins of the UAC family are encoded within operons that also encode ureases or amidases. The putative permeases (the AmiS proteins) may therefore catalyze transport of urea and short chain aliphatic amides such as acetamide. They are of 171-206 amino acyl residues in length and possess 6 putative transmembrane α-helical spanners (TMSs). AmiS homologues are found in both Gram-positive bacteria and Gram-negative bacteria. ATP-binding proteins (AmiB) homologous to Clp proteases are found within some but not all of these operons. AmiB has been proposed to function to energize transport, but no direct evidence supports this proposal.
UreI homologues of Helicobacter pylori and H. hepaticus have been functionally characterized (Weeks and Sachs, 2001; Weeks et al., 2004). These 6 TMS proteins of 195 and 170 amino acyl residues form H -gated urea channels (Sachs et al., 2006). A periplasmic histidine, his 123, is essential for H stimulation of channel activity of the H. pylori protein while several residues including His-50 (boundary of TMS2), Glu-56 (1st periplasmic loop), Asp-59 (boundary of TMS3) and His-70 (boundary of TMS6) affect proton gating in the H. hepaticus protein. Thus, gating is due to periplasmically localized residues that control the channel conformation.
UreI-mediated urea transport is urea specific, passive, nonsaturable, relatively temperature-independent and nonelectrogenic. It is the H -gated urea channel that regulates cytoplasmic urease, the enzyme that allows survival and colonization of the stomach by H. pylori.
H. pylori causes gastritis, gastric ulcers and gastric adenocarcinoma. Its proton-gated inner-membrane urea channel, HpUreI, is essential for survival in the acidic environment of the stomach. The channel is closed at neutral pH and opens at acidic pH to allow the rapid access of urea to cytoplasmic urease. Urease produces NH3 and CO2, neutralizing entering protons and thus buffering the periplasm to a pH of roughly 6.1, even in gastric juice at a pH below 2.0. Strugatsky et al. (2012) reported the structure of HpUreI, revealing six protomers assembled in a hexameric ring surrounding a central bilayer plug of ordered lipids. Each protomer encloses a channel formed by a twisted bundle of six transmembrane helices. The bundle defines a previously unobserved fold comprising a two-helix hairpin motif repeated three times around the central axis of the channel, without the inverted repeat of mammalian-type urea transporters. Both the channel and the protomer interface contain residues conserved in the AmiS/UreI superfamily, suggesting the preservation of channel architecture and oligomeric state in this superfamily. Predominantly aromatic or aliphatic side chains line the entire channel and define two consecutive constriction sites in the middle of the channel. Mutation of Trp%u2009153 in the cytoplasmic constriction site to Ala or Phe decreased the selectivity for urea in comparison with thiourea, suggesting that solute interaction with Trp%u2009153 contributes specificity. This hexameric channel structure provides a model for the permeation of urea and small amides in prokaryotes (Strugatsky et al. 2012).
The transport reaction catalyzed by homologues of the UAC family is:
urea (out) or amide (out) %u2192 urea (in) or amide (in)
Putative amide transporter (AmiS) (Wilson et al., 1995).
AmiS of Pseudomonas aeruginosa
Proton-gated urea transport channel (UreI) (pH-sensitive). Allows the transmembrane flow of urea, hydroxyurea and (at a low rate) water. KB for urea is ~150mM (Sachs et al., 2006; Scott et al., 2010). Transport kinetics and selectivity have been defined (Gray et al., 2011). The 3-d structure reveals a hexameric protein with a channel included within the twisted 6 TMS bundle of each protomer. It displays a two helix hairpin structure repeated three times around the central axis of the channel (Strugatsky et al. 2012).
UreI of Helicobacter pylori
The hexameric ring urea/acetamide/small amide channel, UreI (7 TMSs) (Huysmans et al., 2012).
UreI of Bacillus cereus (Q814I5)