Sites of pH regulation of the urea channel of Helicobacter pylori

Helicobacter pylori (Hp) and Streptococcus salivarius (Ss) require intrabac terial urease for acid resistance and express a urea channel, UreI. The pre sence of UreI was shown to increase urea permeability approximate to 300-fo ld over that of a non-polar ureI deletion mutant. Expression of SsUreI in X enopus oocytes increased urea uptake pH independently, whereas HpUreI shows an acidic pH dependence, half-maximal at pH 6.0. Mutagenesis of all histid ines, aspartates, glutamates and the lysine in the periplasmic domain of Hp UreI showed that His-123, His-131, Asp-129, Asp-140, Glu-138 and Lys-132 in the second periplasmic loop (PL2) and His-193 in the C-terminus (Ct) were important for activation of transport. With the exception of a lysine that was shown to substitute for His-193 in HpUreI, these charged amino acids ar e absent in SsUreI. A chimera in which PL1 of HpUreI was replaced by PL1 of SsUreI retained activity at acidic pH and gained partial activity at neutr al pH. Exchange of PL2 inactivated transport, whereas exchange of Ct had no effect. Chimeras, in which either PL1 or PL2 of HpUreI replaced those of S sUreI, retained wild-type transport, but replacement of the Ct or both loop s inactivated transport. PL1 appears to be important for restricting transp ort through HpUreI at neutral pH, whereas protonation of three histidines i n PL2 and Ct and the presence of three dicarboxylic amino acids in PL2 appe ars to be necessary to activate HpUreI at acidic pH.