Chlamydia trachomatis Polymorphic Membrane Proteins (Pmps) Form Functional Homomeric and Heteromeric Oligomers.

Alison Favaroni,Johannes H Hegemann

doi:10.3389/fmicb.2021.709724

Alison Favaroni, Johannes H Hegemann

Open Access

https://doi.org/10.3389/fmicb.2021.709724

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Abstract

Chlamydiae are Gram-negative, obligate intracellular bacteria, which infect animals and humans. Adhesion to host cells, the first step in the infection process, is mediated by polymorphic membrane proteins (Pmps). Pmps constitute the largest chlamydial protein family, with 9 members (subdivided into six subtypes) in C. trachomatis and 21 in C. pneumoniae, and are characterized by the presence of multiple copies of GGA(I,L,V) and FxxN motifs. Motif-rich fragments of all nine C. trachomatis Pmps act as adhesins and are essential for infection. As autotransporters, most Pmp proteins are secreted through their β-barrel domain and localize on the surface of the chlamydial cell, where most of them are proteolytically processed. Classical autotransporters are monomeric proteins, which can function as toxins, proteases, lipases and monoadhesive adhesins. Here we show that selected recombinant C. trachomatis Pmp fragments form functional adhesion-competent multimers. They assemble into homomeric and heteromeric filaments, as revealed by non-denaturing gel electrophoresis, size-exclusion chromatography and electron microscopy. Heteromeric filaments reach 2 μm in length, significantly longer than homomeric structures. Filament formation was independent of the number of motifs present in the fragment(s) concerned and their relative affinity for host cells. Our functional studies demonstrated that only adhesion-competent oligomers were able to block a subsequent infection. Pre-loading of infectious chlamydial cells with adhesion-competent Pmp oligomers maintained the subsequent infection, while adhesion-incompetent structures reduced infectivity, presumably by blocking the function of endogenous Pmps. The very large number of possible heteromeric and homomeric Pmp complexes represents a novel mechanism to ensure stable adhesion and possibly host cell immune escape.

Highlights

Chlamydiae are Gram-negative bacteria that replicate only within eukaryotic cells and can cause serious infections in humans and other animals (Elwell et al, 2016)
Fragments of PmpA (A408−608) and PmpG (G401−726) were selected as representative of motif-poor polymorphic membrane proteins (Pmps) protein fragments, comparable to the motif-poor fragment analyzed for C. pneumoniae Pmp21 (Luczak et al, 2016)
These proteins have been selected as representative for the different Pmp fragments, which have been predicted to be produced in vivo, harboring different numbers of motifs and representing different portions of the functional passenger domains (PD) (Saka et al, 2011)

Summary

Introduction

Chlamydiae are Gram-negative bacteria that replicate only within eukaryotic cells and can cause serious infections in humans and other animals (Elwell et al, 2016). C. trachomatis is the leading cause of sexually transmitted diseases and of preventable blindness in humans worldwide. Serovars A-C infect the ocular epithelium and recurrent infections may lead to trachoma formation. Serovars D-K target the urogenital tract and in 75% of young women the infection is asymptomatic. Infection is initiated by the adhesion of the infectious elementary body (EB) to target cells. The RBs are able to replicate within the inclusion and eventually redifferentiate into infectious EBs, which leave the cell by lysis or extrusion to begin a new round of infection (Elwell et al, 2016; Christensen et al, 2019). Adhesion of the EB to the host cell is the first and essential step for the establishment of the infection and is a multifactorial process, mediated by an unknown number of adhesins (Hegemann and Moelleken, 2012)

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