Structural basis for substrate selection by the translocation and assembly module of the β-barrel assembly machinery

5 years ago

Structural basis for substrate selection by the translocation and assembly module of the β-barrel assembly machinery

Takoya Shiota, James C. Gumbart, Hyea Hwang, Richard Zimmerman, Karl Lundquist, Mathew J. Belousoff, Trevor Lithgow, Martin Jung, Robert J. A. Goode, Chaille T. Webb, Ralf B. Schittenhelm, Christopher J. Stubenrauch, Rebecca S. Bamert

The assembly of proteins into bacterial outer membranes is a key cellular process that we are only beginning to understand, mediated by the β-barrel assembly machinery (BAM). Two crucial elements of that machinery are the core BAM complex and the translocation and assembly module (TAM), with each containing a member of the Omp85 superfamily of proteins: BamA in the BAM complex, TamA in the TAM. Here, we used the substrate protein FimD as a model to assess the selectivity of substrate interactions for the TAM relative to those of the BAM complex. A peptide scan revealed that TamA and BamA bind the β-strands of FimD, and do so selectively. Chemical cross-linking and molecular dynamics are consistent with this interaction taking place between the first and last strand of the TamA barrel domain, providing the first experimental evidence of a lateral gate in TamA: a structural element implicated in membrane protein assembly. We suggest that the lateral gates in TamA and BamA provide different environments for substrates to engage, with the differences observed here beginning to address how the TAM can be more effective than the BAM complex in the folding of some substrate proteins. The assembly of proteins into the outer membranes is a key process in bacteria. The folding and assembly of these proteins is mediated by a system referred to as the β-barrel assembly machinery (BAM), which includes two related proteins: BamA in the BAM complex and TamA in the TAM. We sought to distinguish these two ostensibly similar proteins and to understand why both have had to be conserved through evolution.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/mmi.13757