Abstract
Transmembrane proteins in the outer membrane of Gram-negative bacteria are almost exclusively β-barrels. They are inserted into the outer membrane by a conserved and essential protein complex called the BAM (for β-barrel assembly machinery). In this commentary, we summarize current research into the mechanism of this protein complex and how it relates to type V secretion. Type V secretion systems are autotransporters that all contain a β-barrel transmembrane domain inserted by BAM. In type Vc systems, this domain is a homotrimer. We argue that none of the current models are sufficient to explain BAM function particularly regarding type Vc secretion. We also find that current models based on the well-studied model system Escherichia coli mostly ignore the pronounced differences in BAM composition between different bacterial species. We propose a more holistic view on how all OMPs, including autotransporters, are incorporated into the lipid bilayer.
Highlights
The term ―autotransporter‖ was coined originally by Thomas Meyer and coworkers [1], based on earlier work from the same group showing that IgA protease from Neisseria is secreted to the bacterial cell surface through the outer membrane apparently without the help of any other factors
We find that current models based on the well-studied model system Escherichia coli mostly ignore the pronounced differences in BAM composition between different bacterial species
While the ―classical‖ autotransporters, with IgA protease from Neisseria being the prototype, were studied in much detail to understand the mechanism of secretion, it soon became apparent that there is a whole set of protein families with similar properties in Gram-negative bacteria, collectively termed Type V secretion systems [5,6]
Summary
The term ―autotransporter‖ was coined originally by Thomas Meyer and coworkers [1], based on earlier work from the same group showing that IgA protease from Neisseria is secreted to the bacterial cell surface through the outer membrane apparently without the help of any other factors. Current research strongly suggests that the term ―autotransporters‖ for the type V secretion systems is misleading, and that these proteins rely on multiple external factors for their export to the bacterial cell surface, for different steps on the way. These factors include the aforementioned Sec machinery for inner membrane translocation, periplasmic chaperones such as SurA, Skp and others that have been shown to keep autotransporters in an export-competent state in the periplasm, and last but not least the -barrel assembly machinery (―BAM‖), one of the few essential outer membrane components in Gram-negative bacteria that catalyzes the membrane insertion of practically all outer membrane -barrel proteins, which includes the autotransporters
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