Investigation of the role of the BAM complex and SurA chaperone in outer-membrane protein biogenesis and type III secretion system expression in Salmonella

Abstract

In Escherichia coli, the assembly of outer-membrane proteins (OMP) requires the BAM complex and periplasmic chaperones, such as SurA or DegP. Previous work has suggested a potential link between OMP assembly and expression of the genes encoding type-III secretion systems. In order to test this hypothesis, we studied the role of the different lipoproteins of the BAM complex (i.e. BamB, BamC, BamD and BamE), and the periplasmic chaperones SurA and DegP, in these two phenotypes in Salmonella. Analysis of the corresponding deletion mutants showed that, as previously described with the DbamB mutant, BamD, SurA and, to a lesser extent, BamE play a role in outer-membrane biogenesis in Salmonella Enteritidis, while the membrane was not notably disturbed in DbamC and DdegP mutants. Interestingly, we found that BamD is not essential in Salmonella, unlike its homologues in Escherichia coli and Neisseria gonorrhoeae. In contrast, BamD was the only protein required for full expression of T3SS-1 and flagella, as demonstrated by transcriptional analysis of the genes involved in the biosynthesis of these T3SSs. In line with this finding, bamD mutants showed a reduced secretion of effector proteins by these T3SSs, and a reduced ability to invade HT-29 cells. As DsurA and DbamE mutants had lower levels of OMPs in their outer membrane, but showed no alteration in T3SS-1 and flagella expression, these results demonstrate the absence of a systematic link between an OMP assembly defect and the downregulation of T3SSs in Salmonella; therefore, this link appears to be related to a more specific mechanism that involves at least BamB and BamD.

The BAM complex subunit BamE (SmpA) is required for membrane integrity, stalk growth and normal levels of outer membrane β-barrel proteins in Caulobacter crescentus

“…It is therefore difficult to make broad statements about the effects of individual chemicals on the loss of BamE/OmlA. However, we noted two trends: (i) whether the species in question was growthinhibited by non-ionic detergents or not, the bamE/omlA mutant of the same species was not affected to a greater extent than the wild-type (Fuangthong et al, 2008;Lewis et al, 2008;Ochsner et al, 1999); and (ii) all species tested were growth-inhibited by rifampicin, and all bamE/omlA mutants were more sensitive to rifampicin than the wildtype strains (Fardini et al, 2009;Lewis et al, 2008;Ochsner et al, 1999). Chemical sensitivity is influenced by the specific complement of porins and outer membrane transporters present in each wild-type strain, as well as which molecules are affected by the loss of BamE/OmlA, so it is not surprising to find distinct phenotypes in different species.…”

“…5d, lanes 1 and 2). Unlike the P. aeruginosa omlA and S. Enteritidis bamE mutants that have little or no change in their OMP profiles (Fardini et al, 2009;Ochsner et al, 1999), DbamE membranes were severely deficient in three prominent, high molecular mass proteins (Fig. 5d, lanes 3 and 4).…”

“…When proteins of the BAM complex are removed from N. meningitidis, E. coli or S. Enteritidis by mutation or depletion, OMPs of various types are reduced in abundance or accumulate in premature forms (Fardini et al, 2009;Sklar et al, 2007;Voulhoux et al, 2003;Wu et al, 2005). Depletion of BamA or BamD causes profound effects on OMP biogenesis, as only one OMP, PulD, has been reported to be normally assembled when BamA is depleted (Collin et al, 2007).…”

“…Mutants in other species lacking components of the BAM complex are hypersensitive to some chemical agents due to membrane permeability defects (Fardini et al, 2009;Lewis et al, 2008;Ochsner et al, 1999;Ruiz et al, 2006;Volokhina et al, 2009). We tested the sensitivity of the wild-type and DbamE Caulobacter strains to a variety of detergents and antibiotics.…”

“…Each species whose bamE/omlA mutant has been tested for chemical sensitivity defects displays a different basal level of sensitivity to different detergents and antibiotics (Fardini et al, 2009;Fuangthong et al, 2008;Lewis et al, 2008;Ochsner et al, 1999). For example, wild-type Xanthomonas campestris pv.…”