The Helicobacter pylori Urel protein: role in adaptation to acidity and identification of residues essential for its activity and for acid activation

Helicobacter pylori is a human gastric pathogen that survives the strong ac idity of the stomach by virtue of its urease activity. This activity produc es ammonia, which neutralizes the bacteria] microenvironment. Urel, an inne r membrane protein, is essential for resistance to low pH and for the gastr ic colonization of mice by H. pylori. In the heterologous Xenopus oocytes e xpression system, Urel behaves like an H+-gated urea channel, and His-123 w as found to be important for low pH activation. We investigated the role of Urel directly in H. pylori and showed that, in the presence of urea, strai ns expressing wild-type Urel displayed very rapid stimulation of extracellu lar ammonia production upon exposure to pH less than or equal to 5. This re sponse was not observed when acetamide was used as a source of ammonia; the refore, it is specific for urea hydrolysis. To identify residues critical f or Urel activity or activation, we constructed H. pylori strains carrying i ndividual chromosomal mutations of Urel (I) in the four conserved histidine residues (H71, H123, H131, H193) and (ij) in a conserved region of the thi rd intracellular loop (L165, G166, K167, F168). The distal H193 (and not H1 23) was found to be crucial for stimulating the production of ammonia at lo w pH; a single mutation in this residue uncoupled the Urel activity from it s acid activation. The third intracellular loop of Urel was shown to be imp ortant for Urel activity. Thus, in H. pylori, Urel is necessary for the ada ptation of urease activity to the extracellular pH. Urel behaves like a nov el type of urea transporter, and the identification of residues essential f or its function in H. pylori provides new insight into the unusual molecula r mechanism of low pH activation.