Mechanical forces regulate the reactivity of a thioester bond in a bacterial adhesin

Abstract

Bacteria must withstand large mechanical shear forces when adhering to and colonizing hosts. Recent structural studies on a class of Gram-positive bacterial adhesins have revealed an intramolecular Cys-Gln thioester bond that can react with surfaceassociated ligands to covalently anchor to host surfaces. Two other examples of such internal thioester bonds occur in certain anti-proteases and in the immune complement system, both of which react with the ligand only after the thioester bond is exposed by a proteolytic cleavage. We hypothesized that mechanical forces in bacterial adhesion could regulate thioester reactivity to ligand analogously to such proteolytic gating. Studying the pilus tip adhesin Spy0125 of Streptococcus pyogenes, we developed a single molecule assay to unambiguously resolve the state of the thioester bond. We found that when Spy0125 was in a folded state, its thioester bond could be cleaved with the small-molecule nucleophiles methylamine and histamine, but when Spy0125 was mechanically unfolded and subjected to forces of 50 -350 piconewtons, thioester cleavage was no longer observed. For folded Spy0125 without mechanical force exposure, thioester cleavage was in equilibrium with spontaneous thioester reformation, which occurred with a half-life of several minutes. Functionally, this equilibrium reactivity allows thioester-containing adhesins to sample potential substrates without irreversible cleavage and inactivation. We propose that such reversible thioester reactivity would circumvent potential soluble inhibitors, such as histamine released at sites of inflammation, and allow the bacterial adhesin to selectively associate with surface-bound ligands.Thioester bonds are ubiquitous in biology and are commonly employed as reactive intermediates in metabolic pathways, including ubiquitinylation (1), fatty acid synthesis (2), and nonribosomal peptide synthesis (3). In addition, there are two rare but notable examples of intramolecular thioester bonds that form between Cys and Gln/Glu side chains: in the ␣2-macroglobulin (A2M) 2 anti-proteases and in the C3 and C4 proteins of the immune complement system (4 -6). In both cases, the thioester bond functions as the electrophilic substrate to draw a nucleophilic ligand and create an intermolecular covalent bond with the target (7). Moreover, both A2Ms and the immune complement proteins utilize a proteolytic gating mechanism to regulate thioester reactivity; the thioester bond is protected and buried in the hydrophobic core until a specific proteolytic cleavage unmasks the bond to react with local nucleophiles (7). For example, the A2M anti-protease acts through a "Venus flytrap mechanism" whereby a bait region of A2M attracts a protease that subsequently cleaves A2M, exposing the thioester to irreversibly react with and inactivate the protease (4, 6). For C3, proteolysis occurs at target surfaces by upstream effectors of the immune complement pathway. Once cleaved, C3 readily reacts with ligands on the microbial cell with a half-life of microsecond...

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Protein folding modulates the adhesion strategy of Gram positive pathogens

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