title: High-salinity growth conditions promote Tat-independent secretion of Tat substrates in Bacillus subtilis.
abstract: The Gram-positive bacterium Bacillus subtilis contains two Tat translocases, which can facilitate transport of folded proteins across the plasma membrane. Previous research has shown that Tat-dependent protein secretion in B. subtilis is a highly selective process and that heterologous proteins, such as the green fluorescent protein (GFP), are poor Tat substrates in this organism. Nevertheless, when expressed in Escherichia coli, both B. subtilis Tat translocases facilitated exclusively Tat-dependent export of folded GFP when the twin-arginine (RR) signal peptides of the E. coli AmiA, DmsA, or MdoD proteins were attached. Therefore, the present studies were aimed at determining whether the same RR signal peptide-GFP precursors would also be exported Tat dependently in B. subtilis. In addition, we investigated the secretion of GFP fused to the full-length YwbN protein, a strict Tat substrate in B. subtilis. Several investigated GFP fusion proteins were indeed secreted in B. subtilis, but this secretion was shown to be completely Tat independent. At high-salinity growth conditions, the Tat-independent secretion of GFP as directed by the RR signal peptides from the E. coli AmiA, DmsA, or MdoD proteins was significantly enhanced, and this effect was strongest in strains lacking the TatAy-TatCy translocase. This implies that high environmental salinity has a negative influence on the avoidance of Tat-independent secretion of AmiA-GFP, DmsA-GFP, and MdoD-GFP. We conclude that as-yet-unidentified control mechanisms reject the investigated GFP fusion proteins for translocation by the B. subtilis Tat machinery and, at the same time, set limits to their Tat-independent secretion, presumably via the Sec pathway.
authors.affiliation: Department of Medical Microbiology, University of Groningen and University Medical Center Groningen, Groningen, the Netherlands.
authors.forename: René
authors.initials: R
authors.lastname: van der Ploeg
authors.forename: Carmine G
authors.initials: CG
authors.lastname: Monteferrante
authors.forename: Sjouke
authors.initials: S
authors.lastname: Piersma
authors.forename: James P
authors.initials: JP
authors.lastname: Barnett
authors.forename: Thijs R H M
authors.initials: TR
authors.lastname: Kouwen
authors.forename: Colin
authors.initials: C
authors.lastname: Robinson
authors.forename: Jan Maarten
authors.initials: JM
authors.lastname: van Dijl
mesh_terms: D001412:Bacillus subtilis / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D021382:Protein Sorting Signals; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D054712:Salinity
chemical_list: D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D021382:Protein Sorting Signals; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins