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 acidity of the stomach by virtue of its urease activity. This activity produces ammonia, which neutralizes the bacteria] microenvironment. Urel, an inner membrane protein, is essential for resistance to low pH and for the gastric colonization of mice by H. pylori. In the heterologous Xenopus oocytes expression system, Urel behaves like an H+-gated urea channel, and His-123 was 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, strains expressing wild-type Urel displayed very rapid stimulation of extracellular ammonia production upon exposure to pH less than or equal to 5. This response was not observed when acetamide was used as a source of ammonia; therefore, it is specific for urea hydrolysis. To identify residues critical for Urel activity or activation, we constructed H. pylori strains carrying individual chromosomal mutations of Urel (I) in the four conserved histidine residues (H71, H123, H131, H193) and (ij) in a conserved region of the third intracellular loop (L165, G166, K167, F168). The distal H193 (and not H123) was found to be crucial for stimulating the production of ammonia at low pH; a single mutation in this residue uncoupled the Urel activity from its acid activation. The third intracellular loop of Urel was shown to be important for Urel activity. Thus, in H. pylori, Urel is necessary for the adaptation of urease activity to the extracellular pH. Urel behaves like a novel type of urea transporter, and the identification of residues essential for its function in H. pylori provides new insight into the unusual molecular mechanism of low pH activation.