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.


 

References:

Vollan, H.S., T. Tannæs, D.A. Caugant, G. Vriend, and G. Bukholm. (2017). Outer membrane phospholipase A's roles in Helicobacter pylori acid adaptation. Gut Pathog 9: 36.

Examples:

TC#NameOrganismal TypeExample
1.B.84.1.1

The Outer Membrane Porin/Phospholipase A1 (OMPLA1) of 355 aas and 12 - 14 transmembrane β-strands in a β-barrel having porin properties.  Transports urea, ammonium ions and small molecules with diameters of less than 4 Å. Besides its role as a phospholipase, it lets urea enter and ammonium exit the periplasm, thereby contributing to acid tolerance (Vollan et al. 2017).

 

OMPLA1 of Helicobacter pylori

 
1.B.84.1.2

Outer membrane phospholipase A-like protein, PldA, of 273 aas and 13 predicted transmembrane β-strands in a β-barrel with putative porin activity. 

PldA of Flavobacterium johnsoniae Å (Cytophaga johnsonae)

 
1.B.84.1.3

Putative porin/phospholipase A1 of 252 aas and 10 - 11 β-strands in a β-barrel.

PLase A1 of Vibrio parahaemolyticus

 
1.B.84.1.4

Uncharacterized protein of 406 aas with an-N-terminal TMS followed by a 190 aa long N-terminal hydrophilic domain and a C-terminal putative 11 strainded β-barrel domain. Putative phospholipase A1.

UP of  Sphingobium baderi

 
1.B.84.1.5

Phospholipase A1 of 417 aas and 14 predicted β-strands.

Phospholipase A1 of Acidovorax avenae

 
1.B.84.1.6

Phospholipase A1 of 407 aas and 10 predicted β-strands.

PLA1 of Opitutus terrae

 
1.B.84.1.7

Uncharacterized protein of 316 aas

UP of Elusimicrobia bacterium GWD2_63_28 (subsurface metagenome)

 
1.B.84.1.8

Uncharacterized protein of

UP of Fibrobacter sp. (anaerobic digester metagenome)