Molecular strategy for blocking isopeptide bond formation in nascent pilin proteins Jaime Andrés Rivas-Pardoa,1, Carmen L. Badillaa, Rafael Tapia-Rojoa, Álvaro Alonso-Caballeroa, and Julio M. Fernándeza,1 aDepartment of Biological Sciences, Columbia University, NY 10027 Edited by Yale E. Goldman, Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, PA, and approved August 3, 2018 (received for review May 4, 2018) Bacteria anchor to their host cells through their adhesive pili, (β-strand1) and N303 (β-strand11) (23). This intramolecular co- which must resist the large mechanical stresses induced by the valent bond accounts for the mechanical rigidity of the protein, as host as it attempts to dislodge the pathogens. The pili of gram- the carboxyl terminus of the C-terminal domain cross-links to the positive bacteria are constructed as a single polypeptide made of adjacent pilin subunit at K161, which leaves the majority of the hundreds of pilin repeats, which contain intramolecular isopeptide N-terminal domain outside of the force pathway (23, 24) (Fig. 1A bonds strategically located in the structure to prevent their and SI Appendix,Fig.S1). Hence, the isopeptide bond is re- unfolding under force, protecting the pilus from degradation by sponsible for the resistance of bacterial adhesion to me- chanical shocks, since mechanically labile pilin proteins that extant proteases and oxygen radicals. Here, we demonstrate the – design of a short peptide that blocks the formation of the isopeptide extend under force are vulnerable to degradative processes (30 33). In this work, we demonstrate the design of short peptides bond present in the pilin Spy0128 from the human pathogen Strep- that, by blocking the formation of the critical intramolecular tococcus pyogenes, resulting in mechanically labile pilin domains. isopeptide bonds, readily disrupt the mechanical properties of We use a combination of protein engineering and atomic-force mi- the Spy0128 pilin. We targeted the C-terminal domain of the croscopy force spectroscopy to demonstrate that the peptide blocks Spy0128 pilin protein with a peptide—isopeptide blocker—designed the formation of the native isopeptide bond and compromises the to mimic the β-strand11 of the pilin domain. Our strategy was mechanics of the domain. While an intact Spy0128 is inextensible at simple: by presenting an exogenously added peptide that mimics any force, peptide-modified Spy0128 pilins readily unfold at very β-strand11, we forced emerging Spy0128 pilin proteins to form in- BIOCHEMISTRY low forces, marking the abrogation of the intramolecular isopeptide termolecular isopeptide bonds between K179 (β-strand1) and N14 bond as well as the absence of a stable pilin fold. We propose that in the isopeptide blocker (Fig. 1 B–D). Timing of the intervention is isopeptide-blocking peptides could be further developed as a type critical. The intermolecular isopeptide bond between the of highly specific antiadhesive antibiotics to treat gram-positive Spy0128 pilin and the peptide must take place before the native pathogens. β-strand11 of the emerging pilin can complete the fold and trigger the formation of the native isopeptide bond, after which protein folding | isopeptide bond | protein mechanics | antibiotic peptide | intervention is futile. We demonstrate the crucial nature of this single-molecule force spectroscopy timing by expressing the isopeptide blocker and its target Spy0128 pilins in sequence. Blockage occurs only if the isopep- tide blocker is expressed before Spy0128, resembling an antibi- acterial infections are initiated by the attachment of bacteria otic peptide that would interfere with the shaft protein before its Bto their host cells, which is mediated by specialized fila- mentous structures known as pili (1–3). As these adhesins are Significance recognized virulence factors, pili have emerged as a target for the development of new vaccines and antiadhesives (4–9). In gram- positive bacteria, pili are assembled as a single polypeptide At the onset of an infection, gram-positive bacteria adhere to composed of up to hundreds of pilin protein repeats placed in host cells through their pili, filamentous structures built by tandem (10–12). Pili are known to be important factors in the hundreds of repeats of pilin proteins. These proteins can virulence of gram-positive pathogens. Indeed, failure in the withstand large mechanical challenges without unfolding, remaining anchored to the host and resisting cleavage by proper polymerization and assembly of pili leads to diminished proteases and oxygen radicals present in the targeted tissues. virulence (13–15). This central role of the pili in bacterial viru- The key structural component that gives pilins mechanical lence is largely related to their unique ability to resist the me- resilience are internal isopeptide bonds, strategically placed so chanical challenges caused by the drag forces of mucus flow, that pilins become inextensible structures. We target this bond – A coughing, or sneezing (16 19) (Fig. 1 ). These environmental by designing a blocking peptide that interferes with its for- perturbations can develop nanonewton-scale forces on a single mation during folding. We demonstrate that peptide-modified pilus (16, 20), sufficient to unfold any protein domain stabilized pilins lack mechanical stability and extend at low forces. We through noncovalent bonds, such as networks of hydrogen bonds propose this strategy as a rational design of mechanical anti- (21, 22). However, a key structural element of pilin proteins biotics, targeting the Achilles heel of bacterial adhesion. is the presence of intramolecular isopeptide bonds, which mechanically lock the protein structure and render it inextensible Author contributions: J.A.R.-P., C.L.B., and J.M.F. designed research; J.A.R.-P., C.L.B., and at any force (23–26). The pilus of the human pathogen Strepto- J.M.F. performed research; J.M.F. contributed new reagents/analytic tools; J.A.R.-P., coccus pyogenes (27) is assembled as a long shaft of tandem re- R.T.-R., and J.M.F. analyzed data; and J.A.R.-P., R.T.-R., Á.A.-C., and J.M.F. wrote the paper. peats of the Spy0128 pilin protein, capped by a single adhesin The authors declare no conflict of interest. Spy0125 protein. Such pili can reach several micrometers in This article is a PNAS Direct Submission. length (10, 28, 29). Published under the PNAS license. The Spy0128 pilin is composed of two Ig-like domains arranged 1To whom correspondence may be addressed. Email: [email protected] or in tandem, each containing an isopeptide bond that deflects a [email protected]. stretching force away from the folded domain blocking their ex- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tension (Fig. 1A, Inset). For example, the C terminus Ig-like do- 1073/pnas.1807689115/-/DCSupplemental. main contains an isopeptide bond between the side chains of K179 www.pnas.org/cgi/doi/10.1073/pnas.1807689115 PNAS Latest Articles | 1of6 Downloaded by guest on September 30, 2021 A Flow using fluorescent HaloTag ligands, we can readily identify modi- fied Spy0128 pilin proteins in SDS/PAGE gels. Moreover, we can Iso combine the anchoring capabilities of the HaloTag together with C Base F paramagnetic beads to specifically enrich the modified fraction of N Spy0128 pilin proteins recovered after their coexpression with the S. pyogenes F isopeptide-blocker peptide. Shaft Fig. 2 shows the constructs used in our protein expression Tip experiments: the 39-kDa Halo-tagged isopeptide blocker (Fig. 2A) and the 96-kDa polyprotein I91-Spy0128-I91-Spy0128-I91 (Fig. 2B). Since the isopeptide-blocker peptide must compete Host Cell with the native β-strand11 before the folding and formation of BCDthe native isopeptide bond, a high concentration of the isopep- Tagging Site tide blocker is required before the Spy0128 polyprotein folds. HT K179 Intermolecular Thus, we coexpress selectively each construct using two different HT Isopeptide inducible expression vectors within the same bacteria (Materials N HaloTag HT Bond and Methods). The purified proteins are then labeled with the fluorescent Alexa488 Halo ligand. Fig. 2C shows an SDS/PAGE N14 + gel of the purified proteins demonstrating the labeling of the Spy0128 polyprotein by Alexa488, and its corresponding Coo- N massie blue on the right. Inducing the Spy0128 polyprotein alone or the polyprotein before the peptide does not result in detect- N303 able Alexa488 labeling of the Spy0128 polyprotein (Fig. 2C,I and IA bands). However, when the peptide is expressed before Fig. 1. Molecular strategy for abolishing the mechanical rigidity of isopeptide-stabilized pilins. (A) Gram-positive bacteria adhere to their host cells using micrometer-long pili, assembled as single modular polypeptides, A which must resist shear forces in excess of 1 nN and remain immune to Donor -strand protease digestion. S. pyogenes pili are built as repeats of the pilin protein EFTDKDMTITFTNKKDAE TEV site HaloTag Spy0128, which is composed of two isopeptide-delimited domains (Inset). These isopeptide bonds are critically placed such that they shunt the force vector (blue) away from the structure and prevent the unfolding of the pilin B Spy0128 Spy0128 protein. (B) In the C-terminal domain of Spy0128 (A, Inset: orange), the intramolecular isopeptide bond is formed between K179 and N303. We (His) I91 N I91 N I91 cys-cys engineered a short peptide that is identical to β-strand11, containing an aspar- 6 C C agine residue in position 14 that matches N303 in the original structure and competes for isopeptide formation during translation and folding. In addition, we included a HaloTag at the C-terminal end of the engineered peptide. Al- CD IAIMWIA IAIMWIA though the channel represents the ribosomal tunnel in the experiments that 174 kDa we show here, it can also be understood as the bacterial translocon, through kDa i which pilin proteins are exported to the periplasm of gram-positive bacteria to i 250 ii 150 fold and assemble to form the pilus.