Abstract
Adhesive type 1 pili from enteroinvasive, Gram-negative bacteria mediate attachment to host cells. Up to 3000 copies of the main pilus subunit, FimA, assemble into the filamentous, helical quaternary structure of the pilus rod via a mechanism termed donor-strand complementation, in which the N-terminal extension of each subunit, the donor strand, is inserted into the incomplete immunoglobulin-like fold of the preceding FimA subunit. For FimA from Escherichia coli, it has been previously shown that the protein can also adopt a monomeric, self-complemented conformation in which the donor strand is inserted intramolecularly in the opposite orientation relative to that observed for FimA polymers. Notably, soluble FimA monomers can act as apoptosis inhibitors in epithelial cells after uptake of type 1-piliated pathogens. Here, we show that the FimA orthologues from Escherichia coli, Shigella flexneri, and Salmonella enterica can all fold to form self-complemented monomers. We solved X-ray structures of all three FimA monomers at 0.89–1.69 Å resolutions, revealing identical, intramolecular donor-strand complementation mechanisms. Our results also showed that the pseudo-palindromic sequences of the donor strands in all FimA proteins permit their alternative folding possibilities. All FimA monomers proved to be 50–60 kJ/mol less stable against unfolding than their pilus rod-like counterparts (which exhibited very high energy barriers of unfolding and refolding). We conclude that the ability of FimA to adopt an alternative, monomeric state with anti-apoptotic activity is a general feature of FimA proteins of type 1-piliated bacteria.
Highlights
Adhesive type 1 pili from enteroinvasive, Gram-negative bacteria mediate attachment to host cells
The X-ray structures of FimAECO, FimASHI, and FimASAL demonstrated that the ability of FimA orthologues to adopt a self-complemented, monomeric conformation is based on the pseudo-palindromic nature of their donor strand, and that the register of donor strand insertion is dictated by the two invariant glycines (Gly8 and Gly14) in the donor strand, which are the only residues that can be accommodated by the very shallow pockets P1 and P4 without disrupting the -sheet hydrogen-bonding network between the donor strand and the neighboring strand A and F
We showed that all three FimA orthologues investigated (FimAECO, FimASHI, and FimASAL) were capable of folding to an alternative, self-complemented, monomeric form of only marginal thermodynamic stability that likely represents the FimA species that acts as an inhibitor of host cell apoptosis after internalization of the respective pathogen [21]
Summary
Adhesive type 1 pili from enteroinvasive, Gram-negative bacteria mediate attachment to host cells. In the structure of the pilus rod, the FimA subunits interact via inter-molecular donor strand complementation, in which the N-terminal extension of each subunit, termed the donor strand, inserts as a -strand into the preceding subunit and completes its IG-like fold (Fig. 1B) [11, 13, 14]. In 2010, Sukumaran et al [21] discovered that the FimA proteins from the enteroinvasive pathogens E. coli, Shigella flexneri, and Salmonella enterica have a second function besides formation of homopolymeric pilus rods; soluble forms of FimA, independent of FimC, act as inhibitors of host cell apoptosis after pathogen internalization by stabilizing the interaction between hexokinase and the voltage-dependent anion channel (VDAC) on the surface of mitochondria. We show that the FimA orthologues from all three pathogens can adopt alternative, assembly incompetent, mono-
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
