A small protein with a big role: EscS in type III secretion system

Irit Tseytin ¹ Bosko Mitrovic ¹ Nofar David ¹ Katja Langenfeld ² Andreas Diepold ² Raz Zarivach ³ Neta Sal-Man ¹

¹The Shraga Segal Department of Microbiology, Immunology and Genetics, Ben-Gurion University of the Negev, Israel
²Department of Ecophysiology Karl-Von-Frisch-Str. 10, Max Planck Institute for Terrestrial Microbiology, Germany
³Department of Life Sciences and the National Institute for Biotechnology in the Negev, Ben-Gurion University of the Negev, Israel

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.