Structural and functional characterization of SiiA, an auxiliary protein from the SPI4‐encoded type 1 secretion system from Salmonella enterica. Issue 5 (17th September 2019)

• Record Type: Journal Article
• Authors: Kirchweger, Peter; Weiler, Sigrid; Egerer‐Sieber, Claudia; Blasl, Anna‐Theresa; Hoffmann, Stefanie; Schmidt, Christiane; Sander, Nathalie; Merker, Dorothee; Gerlach, Roman G.; Hensel, Michael; Muller, Yves A.
• Is Part Of: Molecular microbiology. Volume 112:Issue 5(2019)
• Page Start: 1403
• Page End: 1422
• Publication Date: 2019-09-17
• Subjects: Molecular microbiology -- Periodicals; 572.829
• DOI: 10.1111/mmi.14368
• Languages: English
• ISSNs: 0950-382X

Abstract

Summary: Salmonella invasion is mediated by a concerted action of the Salmonella pathogenicity island 4 (SPI4)‐encoded type one secretion system (T1SS) and the SPI1‐encoded type three secretion system (T3SS‐1). The SPI4‐encoded T1SS consists of five proteins (SiiABCDF) and secretes the giant adhesin SiiE. Here, we investigated structure–function relationships in SiiA, a non‐canonical T1SS subunit. We show that SiiA consists of a membrane domain, an intrinsically disordered periplasmic linker region and a folded globular periplasmic domain (SiiA‐PD). The crystal structure of SiiA‐PD displays homology to that of MotB and other peptidoglycan (PG)‐binding domains. SiiA‐PD binds PG in vitro, albeit at an acidic pH, only. Mutation of Arg162 impedes PG binding of SiiA and reduces Salmonella invasion efficacy. SiiA forms a complex with SiiB at the inner membrane (IM), and the observed SiiA‐MotB homology is paralleled by a predicted SiiB‐MotA homology. We show that, similar to MotAB, SiiAB translocates protons across the IM. Mutating Asp13 in SiiA impairs proton translocation. Overall, SiiA shares numerous properties with MotB. However, MotAB uses the proton motif force (PMF) to energize the bacterial flagellum, it remains to be shown how usage of the PMF by SiiAB assists T1SS function and Salmonella invasion. Abstract : SiiA is required for the function of the Salmonella SPI4‐encoded T1SS. Using structural and functional studies, we show that the periplasmic domain of SiiA displays a peptidoglycan (PG)‐binding OmpA‐like fold and that SiiA binds PG in vitro and in vivo, albeit at an acidic pH, only. SiiA in complex with SiiB translocates protons across the inner membrane; however, further studies are needed to understand how SiiA uses the proton motif force to support the T1SS.