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The Staphylococcus aureus type VII protein secretion system (T7SS) plays important roles in virulence and intra-species competition. Here we show that the T7SS in strain RN6390 is activated by supplementing the growth medium with haemoglobin, and its cofactor haemin (haem B). Transcript analysis and secretion assays suggest that activation by haemin occurs at a transcriptional and a post-translational level. Loss of T7 secretion activity by deletion of essC results in upregulation of genes required for iron acquisition. Taken together these findings suggest that the T7SS plays a role in iron homeostasis in at least some S. aureus strains.

Unlabelled: Human pathogens frequently use protein mimicry to manipulate host cells in order to promote their survival. Here we show that the opportunistic pathogen Pseudomonas aeruginosa synthesizes a structural homolog of the human α2-macroglobulin, a large-spectrum protease inhibitor and important player of innate immunity. Small-angle X-ray scattering analysis demonstrated that the fold of P. aeruginosa MagD (PA4489) is similar to that of the human macroglobulin and undergoes a conformational modification upon binding of human neutrophil elastase. MagD synthesis is under the control of a general virulence regulatory pathway including the inner membrane sensor RetS and the RNA-binding protein RsmA, and MagD undergoes cleavage from a 165-kDa to a 100-kDa form in all clinical isolates tested. Fractionation and immunoprecipitation experiments showed that MagD is translocated to the bacterial periplasm and resides within the inner membrane in a complex with three other molecular partners, MagA, MagB, and MagF, all of them encoded by the same six-gene genetic element. Inactivation of the whole 10-kb operon on the PAO1 genome resulted in mislocalization of uncleaved, in trans-provided MagD as well as its rapid degradation. Thus, pathogenic bacteria have acquired a homolog of human macroglobulin that plays roles in host-pathogen interactions potentially through recognition of host proteases and/or antimicrobial peptides; it is thus essential for bacterial defense., Importance: The pathogenesis of Pseudomonas aeruginosa is multifactorial and relies on surface-associated and secreted proteins with different toxic activities. Here we show that the bacterium synthesizes a 160-kDa structural homolog of the human large-spectrum protease inhibitor α2-macroglobulin. The bacterial protein is localized in the periplasm and is associated with the inner membrane through the formation of a multimolecular complex. Its synthesis is coregulated at the posttranscriptional level with other virulence determinants, suggesting that it has a role in bacterial pathogenicity and/or in defense against the host immune system. Thus, this new P. aeruginosa macromolecular complex may represent a future target for antibacterial developments.

Pseudomonas aeruginosa is a Gram-negative bacterium that can inhabit a wide variety of environments and infect different hosts. Human infections by this microbe are nowadays perceived as a major health problem due notably to its multiresistance. The present data set provides the first description of P. aeruginosa inner membrane proteome. To achieve this, we combined efficient separation of membranes from PAO1 reference strain using discontinuous sucrose gradient centrifugation and MS-based proteomic analysis. A core list of 991 nonredundant proteins was established and analyzed in terms of trans-membrane domains, signal peptide, and lipobox sequence prediction. Furthermore, functional insights into membrane-spanning and membrane-associated protein complexes have been explored. All mass spectrometry data have been deposited in the ProteomeXchange with identifier PXD000107., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)