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Structural and functional characterization of SiiA, an auxiliary protein from the SPI4‐encoded type 1 secretion system from Salmonella enterica
Author(s) -
Kirchweger Peter,
Weiler Sigrid,
EgererSieber Claudia,
Blasl AnnaTheresa,
Hoffmann Stefanie,
Schmidt Christiane,
Sander Nathalie,
Merker Dorothee,
Gerlach Roman G.,
Hensel Michael,
Muller Yves A.
Publication year - 2019
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/mmi.14368
Subject(s) - periplasmic space , biology , salmonella enterica , pathogenicity island , secretion , flagellum , salmonella , inner membrane , type vi secretion system , peptidoglycan , protein subunit , microbiology and biotechnology , pilus , bacterial outer membrane , biochemistry , escherichia coli , genetics , bacteria , virulence , gene , mitochondrion
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.