1.B.27 The Helicobacter Outer Membrane Porin (Hop) Family
The Hop family consists of 31 outer membrane proteins in Helicobacter pylori. Five such proteins, HopA-HopE have been shown to function as porins both in bacterial outer membranes and in reconstituted planar bilayer membranes. The HopE protein appears to form a β-barrel with 16 transmembrane amphipathic β-strands. It is the smallest of the characterized Hop family members, but it forms the largest channels with a single channel conductance of 1.5 nS in 1M KCl (Bina et al. 2000).
Helicobacter pylori attaches to gastric tissue through members of a paralogous family of 'Helicobacter outer membrane proteins' (Hops), including adhesins BabA, SabA, HopQ, LabA and HopZ. Hops share a conserved 25 kDa C-terminal region that is thought to form an autotransporter-like transmembrane domain. Hops contain a non-continuous transmembrane domain, composed of seven predicted beta-strands at the C-terminus and one at the N-terminus (Coppens et al. 2018). Folding and outer membrane localization of the C-terminal beta-domain depends on a predicted transmembrane beta-strand within the first 16 N-terminal residues. The N-terminus resides in the periplasm, and the crystal and small angle X-ray scattering structures for the SabA extracellular domain reveal a conserved coiled-coil stem domain that connects to transmembrane beta-strands 1 and 2. Thus, Hop adhesins represent a novel outer membrane protein topology encompassing an OmpA-like 8-stranded beta-barrel that is interrupted by a 15 - 108 kDa domain inserted inside the first extracellular loop. The insertion of large, folded domains in extracellular loops is unprecedented in bacterial outer membrane proteins (Coppens et al. 2018). It is possible that these proteins are autotransporters.
The generalized transport reaction catalyzed by Hop family porins is:
solute (out) solute (periplasm)
References:
large channel porin, HopE (Lienlaf et al. 2010).
Proteobacteria
HopE of Helicobacter pylori
SabA adhesion domain-containing protein of 711 aas. The extracellular domain is a conserved coiled-coil stem domain that connects to transmembrane beta-strands 1 and 2 (Coppens et al. 2018). SabA is 96% identical to LabA of the same organism (Paraskevopoulou et al. 2019).
SabA of Helicobacter pylori
HopQ of 632 aas
HopQ of Helicobacter pylori (Campylobacter pylori)
HopZ of 666 aas
HopZ of Helicobacter pylori (Campylobacter pylori)
Outer membrane porin, AlpA, of 517 aas with 1 N-terminal TMS. It is also indicated as an adhesin.
AlpA of Heilcobacter pylori
Putative outer membrane porin, HopK
ε-Proteobacteria
HopK of Helicobacter pylori
Putative porin, HorG
ε-Proteobacteria
HorG of Helicobacter pylori
Putative porin, HomA
ε-Proteobacteria
HomA of Helicobacter pylori
Outer membrane porin/adhesin, BabA (HopZ) (Peck et al. 1999). Exhibits phase variation (Kennemann et al. 2012).
Proteobacteria
BabA of Helicobacter pylori
Outer membrane porin/adhesion, SabB or HopO of 623 aas (de Jonge et al. 2004).
Proteobacteria
HopO of Helicobacter pylori
Outer membrane porin, HopF of 485 aas
Proteobacteria
HopF of Helicobacter pylori
Outer membrane porin, HopV, of 248 aas (Lienlaf et al. 2010).
Proteobacteria
HopV of Helicobacter pylori
Putative proin, HopK, of 284 aas
Proteobacteria
HopK of Helicobacter cetorum
Putative porin (based on homology) of 222 aas, Hop-2.
Proteobacteria
hop-2 of Helicobacter hepaticus
Putative porin (based on homology) of 209 aas, Omp30.
Proteobacteria
Omp30 of Helicobacter hepaticus
Putative porin of 284 aas.
Proteobacteria
Porin of Helicobacter pylori