Polar Localization of Virulence-Related Esx-1 Secretion in Mycobacteria

Fredric Carlsson,Eric J Brown,Shilpa A Joshi,Linda Rangell,Lalita Ramakrishnan

doi:10.1371/journal.ppat.1000285

Abstract

The Esx-1 (type VII) secretion system is critical for virulence of both Mycobacterium tuberculosis and Mycobacterium marinum, and is highly conserved between the two species. Despite its importance, there has been no direct visualization of Esx-1 secretion until now. In M. marinum, we show that secretion of Mh3864, a novel Esx-1 substrate that remains partially cell wall–associated after translocation, occurred in polar regions, indicating that Esx-1 secretion takes place in these regions. Analysis of Esx-1 secretion in infected host cells suggested that Esx-1 activity is similarly localized in vivo. A core component of the Esx-1 apparatus, Mh3870, also localized to bacterial poles, showing a preference for new poles with active cell wall peptidoglycan (PGN) synthesis. This work demonstrates that the Esx-1 secretion machine localizes to, and is active at, the bacterial poles. Thus, virulence-related protein secretion is localized in mycobacteria, suggesting new potential therapeutic targets, which are urgently needed.

Highlights

Mycobacteria, and in particular M. tuberculosis, represent a major human health problem globally [1]
In a transposon mutagenesis screen we identified an Mh3864insertion mutant by virtue of its smooth colony morphology, which is a common feature of mutants affected in the Esx-1 secretion system (Figure S1A)
The Mh3864::tn mutant was deficient in CFP-10 secretion, and exhibited modestly reduced growth in macrophages compared to wild type M. marinum (Figure S1B and S1C), suggesting roles for Mh3864 in Esx-1 secretion and virulence

Summary

Introduction

Mycobacteria, and in particular M. tuberculosis, represent a major human health problem globally [1]. The Esx-1 secretion system [early secreted antigen 6 kilodaltons (Esat-6) secretion system 1], which is primarily encoded by genes within, and adjacent to, the region of difference 1 (RD1), is a major virulence determinant of both M. tuberculosis and M. marinum, apparently regulating bacterial spread to host cells [2,3,4,5,6,7]. In M. tuberulosis the RD1 locus (rv3871-rv3879c) encodes the canonical Esx-1 substrates Cfp-10 and Esat-6, as well as Rv3871 and Rv3877, two of the three core proteins in the secretory apparatus [3,8]. M. marinum constitutes a highly relevant system in which to study functional aspects of the Esx-1 secretion system, likely to extend to M. tuberculosis

Methods

Results

Discussion

Conclusion