Export of autotransported proteins proceeds through an oligomeric ring shaped by C-terminal domains

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Abstract
An investigation was made into the oligomerization, the ability to form pores and the secretion‐related properties of the 45 kDa C‐terminal domain of the IgA protease (C‐IgAP) from Neisseria gonorrhoeae . This protease is the best studied example of the autotransporters (ATs), a large family of exoproteins from Gram‐negative bacteria that includes numerous virulence factors from human pathogens. These proteins contain an N‐terminal passenger domain that em bodies the secreted polypeptide, while the C‐domain inserts into the outer membrane (OM) and trans locates the linked N‐module into the extracellular medium. Here we report that purified C‐IgAP forms an oligomeric complex of ∼500 kDa with a ring‐like structure containing a central cavity of ∼2 nm diameter that is the conduit for the export of the N‐domains. These data overcome the previous model for ATs, which postulated the passage of the N‐module through the hydrophilic channel of the β‐barrel of each monomeric C‐domain. Our results advocate a secretion mechanism not unlike other bacterial export systems, such as the secretins or fimbrial ushers, which rely on multimeric complexes assembled in the OM.