The Escherichia coli periplasmic peptidyl‐prolyl isomerase (PPIase) SurA is involved in the maturation of outer membrane porins. SurA consists of a substantial N‐terminal region, two iterative parvulin‐like domains and a C‐terminal tail. Here we show that a variant of SurA lacking both parvulin‐like domains exhibits a PPIase‐independent chaperone‐like activity in vitro and almost completely complements the in vivo function of intact SurA. SurA interacts preferentially (>50‐fold) with in vitro synthesized porins over other similarly sized proteins, leading us to suggest that the chaperone‐like function of SurA preferentially facilitates maturation of outer membrane proteins.

The YscC protein of Yersinia enterocolitica is essential for the secretion of anti‐host factors, called Yops, into the extracellular environment. It belongs to a family of outer membrane proteins, collectively designated secretins, that participate in a variety of transport processes. YscC has been shown to exist as a stable oligomeric complex in the outer membrane. The production of the YscC complex is regulated by temperature and is reduced in strains carrying mutations in the yscN‐U operon or in the virG gene. The VirG lipoprotein was shown to be required for efficient targeting of the complex to the outer membrane. Electron microscopy revealed that purified YscC complexes form ring‐shaped structures of ≈20 nm with an apparent central pore. Because of the architecture of the multimer, YscC appears to represent a novel type of channel‐forming proteins in the bacterial outer membrane.

In the pathogen Neisseria meningitidis , a completely lipopolysaccharide (LPS)‐deficient but viable mutant can be obtained by insertional inactivation of the lpxA gene, encoding UDP‐GlcNAc acyltransferase required for the first step of lipid A biosynthesis. To study how outer membrane structure and biogenesis are affected by the absence of this normally major component, inner and outer membranes were separated and their composition analysed. The expression and assembly of integral outer membrane proteins appeared largely unaffected. However, the expression of iron limitation‐inducible, cell surface‐exposed lipoproteins was greatly reduced. Major changes were seen in the phospholipid composition, with a shift towards phosphatidylethanolamine and phosphatidylglycerol species containing mostly shorter chain, saturated fatty acids, one of which was unique to the LPS‐deficient outer membrane. The presence of the capsular polysaccharide turned out to be essential for viability without LPS, as demonstrated by using a strain in which LPS biosynthesis could be switched on or off through a tac promoter‐controlled lpxA gene. Taken together, these results can help to explain why meningococci have the unique ability to survive without LPS.

Attacin is a 20 kDa antibacterial protein, originally isolated from the immune haemolymph of Hyalophora cecropia. It has been demonstrated previously that attacin causes increased permeability of the outer membrane of Escherichia coli and inhibition of outer-membrane protein synthesis at the transcriptional level. This is accompanied by inhibition of growth. Here, LPS is shown to serve as the receptor for attacin and evidence is presented that attacin does not need to enter the cell to exert its activity. The increase in outer-membrane permeability precedes any increase in inner-membrane permeability by at least one generation time (∼ 45 min), and the inhibiting effect of attacin on synthesis of outer-membrane proteins is detectable after only 10 min. It is also shown that attacin causes induction of several stress proteins and increased synthesis of LPS within, respectively, 25 and 60 min of treatment. Based on the results presented, it is proposed that attacin has the unique ability to specifically interfere with synthesis of outer-membrane proteins without entering the inner membrane or cytoplasm.