Abstract
BackgroundProtein PIII is one of the major outer membrane proteins of Neisseria gonorrhoeae, 95% identical to RmpM (reduction modifiable protein M) or class 4 protein of Neisseria meningitidis. RmpM is known to be a membrane protein associated by non-covalent bonds to the peptidoglycan layer and interacting with PorA/PorB porin complexes resulting in the stabilization of the bacterial membrane. The C-terminal domain of PIII (and RmpM) is highly homologous to members of the OmpA family, known to have a role in adhesion/invasion in many bacterial species. The contribution of PIII in the membrane architecture and its role in the interaction with epithelial cells has never been investigated.ResultsWe generated a ΔpIII knock-out mutant strain and evaluated the effects of the loss of PIII expression on bacterial morphology and on outer membrane composition. Deletion of the pIII gene does not cause any alteration in bacterial morphology or sensitivity to detergents. Moreover, the expression profile of the main membrane proteins remains the same for the wild-type and knock-out strains, with the exception of the NG1873 which is not exported to the outer membrane and accumulates in the inner membrane in the ΔpIII knock-out mutant strain.We also show that purified PIII protein is able to bind human cervical and urethral cells and that the ΔpIII knock-out mutant strain has a lower ability to adhere to human cervical and urethral cells.ConclusionHere we demonstrated that the PIII protein does not play a key structural role in the membrane organization of gonococcus and does not induce major effects on the expression of the main outer membrane proteins. However, in the PIII knock-out strain, the NG1873 protein is not localized in the outer membrane as it is in the wild-type strain suggesting a possible interaction of PIII with NG1873. The evidence that PIII binds to human epithelial cells derived from the female and male genital tract highlights a possible role of PIII in the virulence of gonococcus and suggests that the structural homology to OmpA is conserved also at functional level.
Highlights
IntroductionProtein PIII is one of the major outer membrane proteins of Neisseria gonorrhoeae, 95% identical to RmpM (reduction modifiable protein M) or class 4 protein of Neisseria meningitidis
Protein PIII is one of the major outer membrane proteins of Neisseria gonorrhoeae, 95% identical to RmpM or class 4 protein of Neisseria meningitidis
The three major outer membrane proteins of N. gonorrhoeae have been historically denoted as protein I, II and III (PI, PII and PIII) [1,2], with PIII forming a trimer with two molecules of PI [2]; PI and PII have been subsequently described as porin and Opa proteins, respectively [3,4,5]
Summary
Protein PIII is one of the major outer membrane proteins of Neisseria gonorrhoeae, 95% identical to RmpM (reduction modifiable protein M) or class 4 protein of Neisseria meningitidis. PorB and Opa, which undergo, respectively, antigenic and phase variation, PIII is highly conserved and expressed by all pathogenic Neisseriae [6]. This characteristic inter- and intra-strain homogeneity is unique among all the main outer membrane constituents, which, contrary to PIII, evolved a strong variability to escape the immune pressure of the host [7,8]. In that study the F62 Neisseria gonorrhoeae strain knocked-out for the pIII gene resulted to be identical to the wild-type strain in terms of competence, porin activity, protease and antibiotic sensitivity. The mutant had minimal differences in colony morphology and was slightly decreased in growth compared to the parent strain [11]
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