The Role of the Small Export Apparatus Protein, SctS, in the Activity of the Type III Secretion System.

Irit Tseytin,Katja Langenfeld,Raz Zarivach,Nofar David,Bosko Mitrovic,Neta Sal-Man,Andreas Diepold

doi:10.3389/fmicb.2019.02551

Abstract

Many gram-negative pathogens utilize a protein complex, termed the type III secretion system (T3SS), to inject virulence factors from their cytoplasm directly into the host cell. An export apparatus that is formed by five putative integral membrane proteins (SctR/S/T/U/V), resides at the center of the T3SS complex. In this study, we characterized the smallest export apparatus protein, SctS, which contains two putative transmembrane domains (PTMD) that dynamically extract from the inner membrane and adopt a helix-turn-helix structure upon assembly of the T3SS. Replacement of each SctS PTMD with an alternative hydrophobic sequence resulted in abolishment of the T3SS activity, yet SctS self- and hetero-interactions as well as the overall assembly of the T3SS complex were unaffected. Our findings suggest that SctS PTMDs are not crucial for the interactions or the assembly of the T3SS base complex but rather that they are involved in adjusting the orientation of the export apparatus relative to additional T3SS sub-structures, such as the cytoplasmic- and the inner-membrane rings. This ensures the fittings between the dynamic and static components of the T3SS and supports the functionality of the T3SS complex.

Highlights

Gram-negative bacterial pathogens, including strains of Escherichia coli, Yersinia, Shigella, Salmonella, and Pseudomonas cause serious human illness that accounts, annually, for millions of deaths worldwide (Naghavi et al, 2015; Troeger et al, 2017)
The SctR5-SctT proteins were found to be closely associated, whereas the four SctS subunits were peripherally associated around the SctR5-SctT. Positioning this complex within earlier structures of flagella and T3SS basal bodies, suggested that the export apparatus complex is not embedded within the inner membrane but, rather, fits the unoccupied density at the periplasm space that was previously called “cup and socket” (Kuhlen et al, 2018)
EscS, as well as it homologs in other T3SSs, is crucial for T3SS activity and the ability of the bacteria to infect host cells (Deng et al, 2004; Diepold et al, 2011; Yerushalmi et al, 2014; Fabiani et al, 2017; Fukumura et al, 2017; Tseytin et al, 2018b; Wagner et al, 2018)

Summary

Introduction

Gram-negative bacterial pathogens, including strains of Escherichia coli, Yersinia, Shigella, Salmonella, and Pseudomonas cause serious human illness that accounts, annually, for millions of deaths worldwide (Naghavi et al, 2015; Troeger et al, 2017). These pathogens all utilize common transport nano-machines, termed the type III secretion systems (T3SSs), which translocate numerous bacterial effectors into the host cells to establish infection (Buttner, 2012; Gaytan et al, 2016; Deng et al, 2017; Wagner et al, 2018). We will use the unified Sct names in the introduction section and the species-specific names of the proteins of our model organism in the results and discussion sections

Methods

Results

Conclusion