Mapping Mechanostable Pulling Geometries of a Therapeutic Anticalin/CTLA-4 Protein Complex.

We used single-molecule AFM force spectroscopy (AFM-SMFS) in combination with click chemistry to mechanically dissociate anticalin, a non-antibody protein binding scaffold, from its target (CTLA-4), by pulling from eight different anchor residues. We found that pulling on the anticalin from residue 60 or 87 resulted in significantly higher rupture forces and a decrease in koff by 2-3 orders of magnitude over a force range of 50-200 pN. Five of the six internal anchor points gave rise to complexes significantly more stable than N- or C-terminal anchor points, rupturing at up to 250 pN at loading rates of 0.1-10 nN s-1. Anisotropic network modeling and molecular dynamics simulations helped to explain the geometric dependency of mechanostability. These results demonstrate that optimization of attachment residue position on therapeutic binding scaffolds can provide large improvements in binding strength, allowing for mechanical affinity maturation under shear stress without mutation of binding interface residues.

Thiol-Rich fp-6 Controls the Tautomer Equilibrium of Oxidized Dopa in Interfacial Mussel Foot Proteins.

3,4-Dihydroxyphenylalanine (Dopa) is a versatile molecule that enables marine mussels to achieve successful underwater adhesion. However, due to its complicated redox chemistry and vulnerability to oxidation, controlling surface adhesion and cohesion has been a challenging issue to overcome. Foot protein type 6 (fp-6), a thiol-rich interfacial mussel adhesive protein, has been reported as a proteinaceous antioxidant for mussels that helps Dopa maintain surface adhesion ability. In this study, we focused on the role of fp-6 in oxidized Dopa. The effect on the tautomer equilibrium of oxidized Dopa was investigated using recombinant fp-6 (rfp-6) and Dopa-incorporated foot protein type 3 fast variant (drfp-3F), which were produced in bacterial cells. The redox chemistry of Dopa in drfp-3F and the role of rfp-6 were observed using a UV-vis spectrophotometer and a surface forces apparatus (SFA). We discovered that rfp-6 shifts the tautomer equilibrium to ΔDopa as a preferred tautomer for oxidized Dopa in drfp-3F and makes drfp-3F better on underwater surface adhesion.

Influence of Fluorination on Single-Molecule Unfolding and Rupture Pathways of a Mechanostable Protein Adhesion Complex.

We investigated the influence of fluorination on unfolding and unbinding reaction pathways of a mechanostable protein complex comprising the tandem dyad XModule-Dockerin bound to Cohesin. Using single-molecule atomic force spectroscopy, we mapped the energy landscapes governing the unfolding and unbinding reactions. We then used sense codon suppression to substitute trifluoroleucine in place of canonical leucine globally in XMod-Doc. Although TFL substitution thermally destabilized XMod-Doc, it had little effect on XMod-Doc:Coh binding affinity at equilibrium. When we mechanically dissociated global TFL-substituted XMod-Doc from Coh, we observed the emergence of a new unbinding pathway with a lower energy barrier. Counterintuitively, when fluorination was restricted to Doc, we observed mechano-stabilization of the non-fluorinated neighboring XMod domain. This suggests that intramolecular deformation is modulated by fluorination and highlights the differences between equilibrium thermostability and non-equilibrium mechanostability. Future work is poised to investigate fluorination as a means to modulate mechanical properties of synthetic proteins and hydrogels.

Enhanced production of Dopa-incorporated mussel adhesive protein using engineered translational machineries.

Mussel adhesive proteins (MAPs) have great potential as bioglues, particularly in wet conditions. Although in vivo residue-specific incorporation of 3,4-dihydroxyphenylalanine (Dopa) in tyrosine-auxotrophic Escherichia coli cells allows for production of Dopa-incorporated bioengineered MAPs (dMAPs), the low production yield hinders the practical application of dMAPs. This low production yield of dMAPs is due to low translational activity of a noncanonical amino acid, Dopa, in E. coli cells. Herein, to enhance the production yield of dMAPs, we investigated the coexpression of Dopa-recognizing tyrosyl-tRNA synthetases (TyrRSs). To use the Dopa-specific Methanococcus jannaschii TyrRS (MjTyrRS-Dopa), we altered the anticodon of tyrosyl-tRNA amber suppressor into AUA (MjtRNATyrAUA ) to recognize a tyrosine codon (AUA). Co-overexpression of MjTyrRS-Dopa and MjtRNATyrAUA increased the production yield of Dopa-incorporated MAP foot protein type 3 (dfp-3) by 57%. Similarly, overexpression of E. coli TyrRS (EcTyrRS) led to a 72% higher production yield of dfp-3. Even with coexpression of Dopa-recognizing TyrRSs, dfp-3 has a high Dopa incorporation yield (over 90%) compared to ones prepared without TyrRS coexpression.

Coacervation of Interfacial Adhesive Proteins for Initial Mussel Adhesion to a Wet Surface.

Coacervation of mussel adhesive proteins (MAPs) is proposed as a potential strategy that mussels may use during secretion due to their high concentration density, lack of dispersion into seawater, and low interfacial tension. Particularly, coacervations of interfacial MAPs, foot protein type-3 fast variant (fp-3F) and type-5 (fp-5), are important in the initial mussel adhesion process due to the relationship between the easy secretion/surface wetting properties of the coacervate and primer-like surface adhesive role of interfacial MAPs, which directly contact the marine surface. To the best of the authors' knowledge, this is the first report on coacervate formation of major recombinant interfacial MAPs with high charge densities and the highest 3,4-dihydroxyphenylalanine (Dopa) contents. Specifically, salt-induced coacervation of fp-3F is observed at low pH values corresponding to the acidified environment of the distal depression during mussel secretion. In addition, it shows enthalpy driven upper critical solution temperature behavior, possibly relying on bridging interactions between like-charged cationic fp-3Fs including salt-bridge and cation-π/π-π interactions in the presence of specific counterions, supported by Raman spectroscopy. It is believed that this study has broadened the scope of the understanding of coacervation of MAPs and may provide new insight for responsive biomaterial design.

Complex coacervates based on recombinant mussel adhesive proteins: their characterization and applications.

Complex coacervates are a dense liquid phase of oppositely charged polyions formed by the associative separation of a mixture of polyions. Coacervates have been widely employed in many fields including the pharmaceutical, cosmetic, and food industries due to their intriguing interfacial and bulk material properties. More recently, attempts to develop an effective underwater adhesive have been made using complex coacervates that are based on recombinant mussel adhesive proteins (MAPs) due to the water immiscibility of complex coacervates and the adhesiveness of MAPs. MAP-based complex coacervates contribute to our understanding of the physical nature of complex coacervates and they provide a promising alternative to conventional invasive surgical repairs. Here, this review provides an overview of recombinant MAP-based complex coacervations, with an emphasis on their characterization and the uses of such materials for applications in the fields of biomedicine and tissue engineering.

In vivo residue-specific dopa-incorporated engineered mussel bioglue with enhanced adhesion and water resistance.

Misaminoacylation of 3,4-dihydroxyphenylalanine (Dopa) molecules to tRNA(Tyr) by endogenous tyrosyl-tRNA synthetase allowed the quantitative replacement of tyrosine residues with a yield of over 90 % by an in vivo residue-specific incorporation strategy, to create, for the first time, engineered mussel adhesive proteins (MAPs) in Escherichia coli with a very high Dopa content, close to that of natural MAPs. The Dopa-incorporated MAPs exhibited a superior surface adhesion and water resistance ability by assistance of Dopa-mediated interactions including the oxidative Dopa cross-linking, and furthermore, showed underwater adhesive properties comparable to those of natural MAPs. These results propose promising use of Dopa-incorporated engineered MAPs as bioglues or adhesive hydrogels for practical underwater applications.

Engineering an artificial catch bond using mechanical anisotropy.

Catch bonds are a rare class of protein-protein interactions where the bond lifetime increases under an external pulling force. Here, we report how modification of anchor geometry generates catch bonding behavior for the mechanostable Dockerin G:Cohesin E (DocG:CohE) adhesion complex found on human gut bacteria. Using AFM single-molecule force spectroscopy in combination with bioorthogonal click chemistry, we mechanically dissociate the complex using five precisely controlled anchor geometries. When tension is applied between residue #13 on CohE and the N-terminus of DocG, the complex behaves as a two-state catch bond, while in all other tested pulling geometries, including the native configuration, it behaves as a slip bond. We use a kinetic Monte Carlo model with experimentally derived parameters to simulate rupture force and lifetime distributions, achieving strong agreement with experiments. Single-molecule FRET measurements further demonstrate that the complex does not exhibit dual binding mode behavior at equilibrium but unbinds along multiple pathways under force. Together, these results show how mechanical anisotropy and anchor point selection can be used to engineer artificial catch bonds.

Integrating Dynamic Network Analysis with AI for Enhanced Epitope Prediction in PD-L1:Affibody Interactions.

Understanding binding epitopes involved in protein-protein interactions and accurately determining their structure is a long standing goal with broad applicability in industry and biomedicine. Although various experimental methods for binding epitope determination exist, these approaches are typically low throughput and cost intensive. Computational methods have potential to accelerate epitope predictions, however, recently developed artificial intelligence (AI)-based methods frequently fail to predict epitopes of synthetic binding domains with few natural homologs. Here we have developed an integrated method employing generalized-correlation-based dynamic network analysis on multiple molecular dynamics (MD) trajectories, initiated from AlphaFold2 Multimer structures, to unravel the structure and binding epitope of the therapeutic PD-L1:Affibody complex. Both AlphaFold2 and conventional molecular dynamics trajectory analysis alone each proved ineffectual in differentiating between two putative binding models referred to as parallel and perpendicular. However, our integrated approach based on dynamic network analysis showed that the perpendicular mode was significantly more stable. These predictions were validated using a suite of experimental epitope mapping protocols including cross linking mass spectrometry and next-generation sequencing-based deep mutational scanning. Our research highlights the potential of deploying dynamic network analysis to refine AI-based structure predictions for precise predictions of protein-protein interaction interfaces.

Engineering the Mechanical Stability of a Therapeutic Affibody/PD-L1 Complex by Anchor Point Selection.

Protein-protein complexes can vary in mechanical stability depending on the direction from which force is applied. Here we investigated the anisotropic mechanical stability of a molecular complex between a therapeutic non-immunoglobulin scaffold called Affibody and the extracellular domain of the immune checkpoint protein PD-L1. We used a combination of single-molecule AFM force spectroscopy (AFM-SMFS) with bioorthogonal clickable peptide handles, shear stress bead adhesion assays, molecular modeling, and steered molecular dynamics (SMD) simulations to understand the pulling point dependency of mechanostability of the Affibody:(PD-L1) complex. We observed diverse mechanical responses depending on the anchor point. For example, pulling from residue #22 on Affibody generated an intermediate unfolding event attributed to partial unfolding of PD-L1, while pulling from Affibody's N-terminus generated force-activated catch bond behavior. We found that pulling from residue #22 or #47 on Affibody generated the highest rupture forces, with the complex breaking at up to ~ 190 pN under loading rates of ~104-105 pN/sec, representing a ~4-fold increase in mechanostability as compared with low force N-terminal pulling. SMD simulations provided consistent tendencies in rupture forces, and through visualization of force propagation networks provided mechanistic insights. These results demonstrate how mechanostability of therapeutic protein-protein interfaces can be controlled by informed selection of anchor points within molecules, with implications for optimal bioconjugation strategies in drug delivery vehicles.

A comparative study on the bulk adhesive strength of the recombinant mussel adhesive protein fp-3.

Mussel adhesive protein (MAP) type 3 (fp-3) is considered one of the key components for mussel adhesion. However, its bulk adhesive strength has not been characterized due to its availability in limited quantities. In the present work, a feasible production (~47 mg l(-1)) of recombinant fp-3 was achieved, and its bulk adhesive strength was measured for the first time; ~0.57 MPa for the unmodified form and ~0.94 and ~2.28 MPa for the 3,4-dihydroxy-L-phenylalanine (DOPA)-modified form, having a 9.6% yield without and with oxidant treatment, respectively. Furthermore, values for the bulk adhesive strength of several DOPA-modified recombinant MAPs were compared. The maximum adhesive strength of DOPA-modified fp-3 after oxidant treatment was stronger than that of type 5 (fp-5), which has a 6.2% modification yield, and was comparable to that of hybrid types fp-131 and fp-151, which have similar yields (~5%). The strong bulk adhesive property of recombinant fp-3 demonstrates its potential use as a promising bioadhesive.

In vivo modification of tyrosine residues in recombinant mussel adhesive protein by tyrosinase co-expression in Escherichia coli.

BACKGROUND: In nature, mussel adhesive proteins (MAPs) show remarkable adhesive properties, biocompatibility, and biodegradability. Thus, they have been considered promising adhesive biomaterials for various biomedical and industrial applications. However, limited production of natural MAPs has hampered their practical applications. Recombinant production in bacterial cells could be one alternative to obtain useable amounts of MAPs, although additional post-translational modification of tyrosine residues into 3,4-dihydroxyphenyl-alanine (Dopa) and Dopaquinone is required. The superior properties of MAPs are mainly attributed to the introduction of quinone-derived intermolecular cross-links. To solve this problem, we utilized a co-expression strategy of recombinant MAP and tyrosinase in Escherichia coli to successfully modify tyrosine residues in vivo. RESULTS: A recombinant hybrid MAP, fp-151, was used as a target for in vivo modification, and a dual vector system of pET and pACYC-Duet provided co-expression of fp-151 and tyrosinase. As a result, fp-151 was over-expressed and mainly obtained from the soluble fraction in the co-expression system. Without tyrosinase co-expression, fp-151 was over-expressed in an insoluble form in inclusion bodies. The modification of tyrosine residues in the soluble-expressed fp-151 was clearly observed from nitroblue tetrazolium staining and liquid-chromatography-mass/mass spectrometry analyses. The purified, in vivo modified, fp-151 from the co-expression system showed approximately 4-fold higher bulk-scale adhesive strength compared to in vitro tyrosinase-treated fp-151. CONCLUSION: Here, we reported a co-expression system to obtain in vivo modified MAP; additional in vitro tyrosinase modification was not needed to obtain adhesive properties and the in vivo modified MAP showed superior adhesive strength compared to in vitro modified protein. It is expected that this co-expression strategy will accelerate the use of functional MAPs in practical applications and can be successfully applied to prepare other Dopa/Dopaquinone-based biomaterials.

Optimal Sacrificial Domains in Mechanical Polyproteins: S. epidermidis Adhesins Are Tuned for Work Dissipation.

The opportunistic pathogen Staphylococcus epidermidis utilizes a multidomain surface adhesin protein to bind host components and adhere to tissues. While it is known that the interaction between the SdrG receptor and its fibrinopeptide target (FgB) is exceptionally mechanostable (∼2 nN), the influence of downstream B domains (B1 and B2) is unclear. Here, we studied the mechanical relationships between folded B domains and the SdrG receptor bound to FgB. We used protein engineering, single-molecule force spectroscopy (SMFS) with an atomic force microscope (AFM), and Monte Carlo simulations to understand how the mechanical properties of folded sacrificial domains, in general, can be optimally tuned to match the stability of a receptor-ligand complex. Analogous to macroscopic suspension systems, sacrificial shock absorber domains should neither be too weak nor too strong to optimally dissipate mechanical energy. We built artificial molecular shock absorber systems based on the nanobody (VHH) scaffold and studied the competition between domain unfolding and receptor unbinding. We quantitatively determined the optimal stability of shock absorbers that maximizes work dissipation on average for a given receptor and found that natural sacrificial domains from pathogenic S. epidermidis and Clostridium perfringens adhesins exhibit stabilities at or near this optimum within a specific range of loading rates. These findings demonstrate how tuning the stability of sacrificial domains in adhesive polyproteins can be used to maximize mechanical work dissipation and serve as an adhesion strategy by bacteria.

Next Generation Methods for Single-Molecule Force Spectroscopy on Polyproteins and Receptor-Ligand Complexes.

Single-molecule force spectroscopy with the atomic force microscope provides molecular level insights into protein function, allowing researchers to reconstruct energy landscapes and understand functional mechanisms in biology. With steadily advancing methods, this technique has greatly accelerated our understanding of force transduction, mechanical deformation, and mechanostability within single- and multi-domain polyproteins, and receptor-ligand complexes. In this focused review, we summarize the state of the art in terms of methodology and highlight recent methodological improvements for AFM-SMFS experiments, including developments in surface chemistry, considerations for protein engineering, as well as theory and algorithms for data analysis. We hope that by condensing and disseminating these methods, they can assist the community in improving data yield, reliability, and throughput and thereby enhance the information that researchers can extract from such experiments. These leading edge methods for AFM-SMFS will serve as a groundwork for researchers cognizant of its current limitations who seek to improve the technique in the future for in-depth studies of molecular biomechanics.

The position of lysine controls the catechol-mediated surface adhesion and cohesion in underwater mussel adhesion.

Intensive studies have found that 3,4-dihydroxyphenylalanine (Dopa) is one of the key molecules for underwater mussel adhesion. Although basic mechanisms of mussel adhesion have been elucidated, little is known about how mussels control the balance between surface adhesion and cohesion, which is critical for successful adhesion without peeling and/or tearing. In this work, we focused on lysine (Lys) molecules which are frequently flanked to Dopa residues in interfacial adhesive proteins, specifically their synergy and anti-synergy on surface adhesion and cohesion. Three model peptides were designed to characterize flanking Lys effects. Through nano-mechanistic analyses, we found that flanking Lys enhanced surface adhesion but disrupted Fe3+-mediated cohesion. Through nuclear magnetic resonance analyses and density functional theory calculations, we corroborated the synergetic effect on surface adhesion and anti-synergetic effect on cohesion. We also confirmed the consistency of flanking Lys effects in the actual protein system. Thus, we, for the first time, discovered that each Dopa molecule in interfacial adhesive proteins is participated in surface adhesion and cohesion differently through controlling the existence of flanking Lys. Our discovery enlightens how nature designs adhesive proteins through according roles of Dopa.

Sprayable Adhesive Nanotherapeutics: Mussel-Protein-Based Nanoparticles for Highly Efficient Locoregional Cancer Therapy.

Following surgical resection for primary treatment of solid tumors, systemic chemotherapy is commonly used to eliminate residual cancer cells to prevent tumor recurrence. However, its clinical outcome is often limited due to insufficient local accumulation and the systemic toxicity of anticancer drugs. Here, we propose a sprayable adhesive nanoparticle (NP)-based drug delivery system using a bioengineered mussel adhesive protein (MAP) for effective locoregional cancer therapy. The MAP NPs could be administered to target surfaces in a surface-independent manner through a simple and easy spray process by virtue of their unique adhesion ability and sufficient dispersion property. Doxorubicin (DOX)-loaded MAP NPs (MAP@DOX NPs) exhibited efficient cellular uptake, endolysosomal trafficking, and subsequent low pH microenvironment-induced DOX release in cancer cells. The locally sprayed MAP@DOX NPs showed a significant inhibition of tumor growth in vivo, resulting from the prolonged retention of the MAP@DOX NPs on the tumor surface. Thus, this adhesive MAP NP-based spray therapeutic system provides a promising approach for topical drug delivery in adjuvant cancer therapy.

Self-assembled adhesive biomaterials formed by a genetically designed fusion protein.

Here we report a recombinant protein (MS) obtained by genetic fusion of a mussel foot protein (Mfp3) motif into a silk spidroin (MaSp1). The MS not only self-assembled into a supramolecular fibre, as does the parent MaSp1, but also showed enhanced adhesiveness resulting from the DOPA-containing Mfp3 portion. The successful incorporation of the wet adhesiveness of Mfp3 into the well-structured assembly of MaSp1 may provide a new insight for the genetic design of underwater adhesive recombinant proteins by utilizing the structural features of a spidroin protein.

A tyrosinase, mTyr-CNK, that is functionally available as a monophenol monooxygenase.

Tyrosinase efficiently catalyzes the ortho-hydroxylation of monophenols and the oxidation of diphenols without any additional cofactors. Although it is of significant interest for the biosynthesis of catechol derivatives, the rapid catechol oxidase activity and inactivation of tyrosinase have hampered its practical utilization as a monophenol monooxygenase. Here, we prepared a functional tyrosinase that exhibited a distinguished monophenolase/diphenolase activity ratio (V max mono/ V max di = 3.83) and enhanced catalytic efficiency against L-tyrosine (k cat = 3.33 ± 0.18 s-1, K m = 2.12 ± 0.14 mM at 20 °C and pH 6.0). This enzyme was still highly active in ice water (>80%), and its activity was well conserved below 30 °C. In vitro DOPA modification, with a remarkably high yield as a monophenol monooxygenase, was achieved by the enzyme taking advantage of these biocatalytic properties. These results demonstrate the strong potential for this enzyme's use as a monophenol monooxygenase in biomedical and industrial applications.

Aquatic proteins with repetitive motifs provide insights to bioengineering of novel biomaterials.

Proteins with repetitive motifs play vital structural and adhesive functions in nature. Some repeat proteins in particular have adapted to harsh aquatic surroundings to support the survival and reproduction of organisms. Significant effort has been made to identify aquatic repeat proteins with attractive properties and functions to be used as novel biomaterials. Examples of such proteins include matrix proteins from pearl oysters, minicollagens from sea anemones, cement proteins from sandcastle worms, and byssal proteins from marine mussels. Here, several repetitive motifs from aquatic proteins are reviewed, and their characteristic properties are linked to practical uses in three aspects of aquatic life: defense, shelter, and attachment. Some repetitive motifs interact with minerals and consequently generate strong outer cover of shells, and some motifs relate with sticky nature, which contribute to organisms' habitation by adhering themselves in harsh aquatic environments. Other motifs, such as silk- or collagen-like motifs, are also involved in structural rigidity as shown in mussel's byssus and egg membrane. Thus, understanding aquatic repetitive motifs will provide clues about biomedical and biotechnological applications of engineered biomaterials in wet environments.