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J Mol Biol. 2011 Sep 30;412(4):553-67. doi: 10.1016/j.jmb.2011.07.035. Epub 2011 Jul 23.

Autotransporter β-domains have a specific function in protein secretion beyond outer-membrane targeting.

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Section of Molecular Microbiology, Department of Molecular Cell Biology, VU University, 1081 HV Amsterdam, The Netherlands.


Autotransporters (ATs) of Gram-negative bacteria contain an N-proximal passenger domain that is transported to the extracellular milieu and a C-terminal β-domain that inserts into the outer membrane (OM) in a β-barrel conformation. This β-domain facilitates translocation of the passenger domain across the OM and has long been considered to be the translocation pore. However, available crystal structures of β-domains show that the β-barrel pore is too narrow for the observed transport of folded elements within the passenger domains. ATs have recently been shown to interact with the β-barrel assembly machinery. These findings questioned a direct involvement of the β-domain in passenger translocation and suggested that it may only target the passenger to the β-barrel assembly machinery pore. To address the function of the β-domain in more detail, we have replaced the β-domain of the Escherichia coli AT hemoglobin protease by β-domains originating from other OM proteins. Furthermore, we have modified the diameter of the β-domain pore. The mutant proteins were analyzed for their capacity to insert into the OM and for surface display of the passenger. Our results show that efficient passenger secretion requires a specific β-domain that not only functions as a targeting device but also is directly involved in the translocation of the passenger to the cell surface.

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