1.B.84. The Outer Membrane Porin/Phospholipase A (OMPLA) Family
The pH of the human gastric mucosa is ~2.5, so that only bacteria with strong acidic stress tolerance can colonize it. The ulcer causing Helicobacter pylori thrives in the gastric mucosa. Vollan et al. 2017 analysed the roles of OMPLA in acid tolerance.It appears to be a twelve stranded beta-barrel with a pore that allows molecules to pass with a diameter up to 4 Å. Structure based multiple sequence alignments revealed the functional roles of many amino acids, and led to the suggestion that OMPLA has multiple functions. Besides its role as a phospholipase, it lets urea enter and ammonium exit the periplasm. A three-dimensional model of OMPLA predicted a transmembrane pore that can aid H. pylori's acid tolerance through urea influx and ammonium efflux. After urea passes through OMPLA into the periplasm, it passes through the pH-gated inner membrane channel, UreI (TC# 1.A.29.1.3) into the cytoplasm where urease hydrolyses it into NH3 and CO2. Most of the NH3 becomes NH4+ that is likely to need an inner membrane channel to reach the periplasm. Two genes that are co-regulated with OMPLA in gastric Helicobacter operons could aid this transport. The NH4+ that might leave the cell through the OMPLA pore has been implicated in H. pylor's pathogenesis (Vollan et al. 2017). This family may belong to the: Outer Membrane Pore-forming Protein I (OMPP-I) Superfamily, but this suggestion needs further substantiation.