Characterisation of the major porins OmpU and OmpT of Vibrio cholerae

Abstract

The asymmetric outer membrane (OM) of a Gram-negative bacterium has many proteins embedded as β-barrel structures in it called outer membrane proteins (OMPs). The majority of these OMPs (porins) form non-selective channels across the OM to allow passive uptake of substrates. The treatment for infections caused by such bacteria mostly involves the administration of drugs/antibiotics, for which these porins play a very crucial role by providing an efficient (although not yet fully understood) route through their channel. The goal of this study is to study small-molecule permeation through the major porins, OmpU and OmpT, of Vibrio cholerae (the causative agent of cholera) for potential use of these proteins as the target for designing antibiotics or vaccines. Towards this project, we have succeeded in solving the 3D X-ray crystal structures of OmpU and OmpT as well as the structures of the major porins from Klebsiella pneumoniae (OmpK36) and Enterobacter cloacae (OmpE36, OmpE35). The proteins (OmpU/T, OmpE35/E36 and OmpK36) show the typical arrangement of porins with three β-barrel monomers arranged into a trimer. Each monomer displays 16 antiparallel β-strands forming the hollow β-barrel formed by 8 long extracellular loops and 8 short periplasmic turns. The latching loop L2 stabilises the trimer while loop L3 departs from the β-barrel fold and constricts the pore half-way through the channel. An unusual feature is observed in the channels of OmpU and OmpT that distinguishes them from other typical porins. In OmpU, the first 10 residues of N-terminus insert into the barrel and constrict the pore. In contrast, the structure of OmpT reveals that the extracellular loop L8 folds inwards to constrict the lumen of the channel. Such constriction elements not only reduce the pore sizes of OmpU and OmpT but may also dramatically affect the internal electrostatics of these channels, which is very important for small-molecule permeation. In addition, we also performed single channel electrophysiology experiments with OmpU and OmpT which revealed interesting features with the addition of carbapenems.