Folding and functions of knotted proteins

Discrepant Effects of Hydrated and Neat Reline on the Conformational Stability of a Knotted Protein

Although knotted proteins are rare in number, their peculiar topology has long intrigued the scientific community. In this study, we have explored the conformational stability of a trefoil-knotted protein, YbeA, in reline (choline chloride:urea in a 1:2 ratio), a well-characterized deep eutectic solvent, using classical molecular dynamics simulation. Deep eutectic solvents (DESs) are explored as a reliable alternative to conventional solvents, effectively altering a protein's structural stability and activity, either stabilizing its native state or disordering its conformation depending on the relevant interactions involved. Here, using pure and hydrated concentrations of reline, we observe the conflicting effect of the DES on the knotted protein's stability. Our studies at room temperature and elevated temperatures show that in pure reline, the protein is conformationally stable and rigid. In contrast, the protein tends to lose its structural integrity in hydrated reline. The stable knotted topology also gets untied as the protein, solvated in hydrated reline, is exposed to an elevated temperature. Using Minimum Distance Distribution Functions and Kirkwood-Buff Integrals, we analyzed the solvation pattern of the DES constituents around the protein. We expect that this study will lead to more effective strategies in developing tailored solvent systems for comprehending the conformational behavior of knotted proteins.

Step-by-Step Self-Assembly of a Double-Walled Knotted Cage With Increasing Topological Complexity

The self-assembly process of a double-walled cage formed from a semiflexible tripodal ligand and a Pd(II) 90-degree block was tracked by NMR and x-ray analysis. At least two intermediate structures with distinct topologies were observed prior to the formation of the final double-walled cage. By optimizing the self-assembly conditions (e.g., time, solvent, and concentration), these topological intermediates were successfully isolated and analyzed by x-ray crystallography. They are considered crucial metastable structures that navigate the shortest pathway to the final structure, demonstrating the critical role of molecular topology in guiding and controlling the kinetics of metal-directed self-assembly.

Upsurge of Spontaneous Knotting in Polar Diblock Active Polymers

Spontaneous formation of knots in long polymers at equilibrium is inevitable but becomes rare in sufficiently short chains. Here, we show that knotting increases by orders of magnitude in diblock polymers having a fraction p of self-propelled monomers. Remarkably, this enhancement is not monotonic in p with an optimal value independent of the monomer's activity. By monitoring the knot's size and position we elucidate the mechanisms of its formation, diffusion, and untying and ascribe the nonmonotonic behavior to the competition between the rate of knot formation and the knot's lifetime. These findings suggest a nonequilibrium mechanism to generate entangled filaments at the nano- and microscales.

Effect of a Knot on the Thermal Stability of Protein MJ0366: Insights into Molecular Dynamics and Monte Carlo Simulations

Protein MJ0366 is a hypothetical protein from Methanocaldococcus jannaschii that has a rare and complex knot in its structure. The knot is a right-handed trefoil knot that involves about half of the protein's residues. In this work, we investigate the thermal stability of protein MJ0366 using numerical simulations based on molecular dynamics and Monte Carlo methods. We compare the results with those of a similar unknotted protein and analyze the effects of the knot on the folding and unfolding processes. We show that the knot in protein MJ0366 increases its thermal stability by creating a topological barrier that prevents the protein from unfolding at high temperatures.

Folding of a tandemly knotted protein: Evidence that a polypeptide chain can get out of deep kinetic traps

It is hard to imagine how proteins can thread and form knots in their polypeptide chains, but they do. These topologically complex structures have challenged the traditional protein folding views of simple funnel-shaped energy landscapes. Previous experimental studies on the folding mechanisms of deeply knotted proteins with a single trefoil knot have yielded evidence that this topology has a more complicated folding landscape than other simpler proteins. However, to date, there have been no attempts to study the folding of any protein in which multiple threading events are needed to create more than one knot within a single polypeptide chain. Here, we report the construction and characterization of an artificial tandemly knotted protein. We find compelling evidence that both domains of the protein form trefoil knots with similar structures and stabilities to the parent single trefoil-knotted protein. In addition, we show that this tandemly knotted protein has a complex folding pathway in which there are additional very slow folding phases that we propose correspond to the formation of the second knot within the system. We also find evidence that during folding this protein gets transiently trapped in deep kinetic traps, however, the majority of protein chains (>90%) manage to partially unfold and acquire the native tandem-knot topology. This work highlights the fact that Nature can tolerate more complex protein topologies than we thought, and despite considerable misfolding during folding, protein chains can find their way to the native state even in the absence of molecular chaperones.

Salt-bridge mediated conformational dynamics in the figure-of-eight knotted ketol acid reductoisomerase (KARI)

The utility of knotted proteins in biological activities has been ambiguous since their discovery. From their evolutionary significance to their functionality in stabilizing the native protein structure, a unilateral conclusion hasn't been achieved yet. While most studies have been performed to understand the stabilizing effect of the knotted fold on the protein chain, more ideas are yet to emerge regarding the interactions in stabilizing the knot. Using classical molecular dynamics (MD) simulations, we have explored the dynamics of the figure-of-eight knotted domain present in ketol acid reductoisomerase (KARI). Our main focus was on the presence of a salt bridge network evident within the knotted region and its role in shaping the conformational dynamics of the knotted chain. Through the potential of mean forces (PMFs) calculation, we have also marked the specific salt bridges that are pivotal in stabilizing the knotted structure. The correlated motions have been further monitored with the help of principal component analysis (PCA) and dynamic cross-correlation maps (DCCM). Furthermore, mutation of the specific salt bridges led to a change in their conformational stability, vindicating their importance.

Everything AlphaFold tells us about protein knots

Recent advances in Machine Learning methods in structural biology opened up new perspectives for protein analysis. Utilizing these methods allows us to go beyond the limitations of empirical research, and take advantage of the vast amount of generated data. We use a complete set of potentially knotted protein models identified in all high-quality predictions from the AlphaFold Database to search for any common trends that describe them. We show that the vast majority of knotted proteins have 31 knot and that the presence of knots is preferred in neither Bacteria, Eukaryota, or Archaea domains. On the contrary, the percentage of knotted proteins in any given proteome is around 0.4%, regardless of the taxonomical group. We also verified that the organism's living conditions do not impact the number of knotted proteins in its proteome, as previously expected. We did not encounter an organism without a single knotted protein. What is more, we found four universally present families of knotted proteins in Bacteria, consisting of SAM synthase, and TrmD, TrmH, and RsmE methyltransferases.

Is There a Functional Role for the Knotted Topology in Protein UCH-L1?

Knotted proteins are present in nature, but there is still an open issue regarding the existence of a universal role for these remarkable structures. To address this question, we used classical molecular dynamics (MD) simulations combined with in vitro experiments to investigate the role of the Gordian knot in the catalytic activity of UCH-L1. To create an unknotted form of UCH-L1, we modified its amino acid sequence by truncating several residues from its N-terminus. Remarkably, we find that deleting the first two N-terminal residues leads to a partial loss of enzyme activity with conservation of secondary structural content and knotted topological state. This happens because the integrity of the N-terminus is critical to ensure the correct alignment of the catalytic triad. However, the removal of five residues from the N-terminus, which significantly disrupts the native structure and the topological state, leads to a complete loss of enzymatic activity. Overall, our findings indicate that UCH-L1's catalytic activity depends critically on the integrity of the N-terminus and the secondary structure content, with the latter being strongly coupled with the knotted topological state.

AlphaKnot 2.0: a web server for the visualization of proteins' knotting and a database of knotted AlphaFold-predicted models

The availability of 3D protein models is rapidly increasing with the development of structure prediction algorithms. With the expanding availability of data, new ways of analysis, especially topological analysis, of those predictions are becoming necessary. Here, we present the updated version of the AlphaKnot service that provides a straightforward way of analyzing structure topology. It was designed specifically to determine knot types of the predicted structure models, however, it can be used for all structures, including the ones solved experimentally. AlphaKnot 2.0 provides the user's ability to obtain the knowledge necessary to assess the topological correctness of the model. Both probabilistic and deterministic knot detection methods are available, together with various visualizations (including a trajectory of simplification steps to highlight the topological complexities). Moreover, the web server provides a list of proteins similar to the queried model within AlphaKnot's database and returns their knot types for direct comparison. We pre-calculated the topology of high-quality models from the AlphaFold Database (4th version) and there are now more than 680.000 knotted models available in the AlphaKnot database. AlphaKnot 2.0 is available at https://alphaknot.cent.uw.edu.pl/.

Knot or not? Identifying unknotted proteins in knotted families with sequence-based Machine Learning model

Knotted proteins, although scarce, are crucial structural components of certain protein families, and their roles continue to be a topic of intense research. Capitalizing on the vast collection of protein structure predictions offered by AlphaFold (AF), this study computationally examines the entire UniProt database to create a robust dataset of knotted and unknotted proteins. Utilizing this dataset, we develop a machine learning (ML) model capable of accurately predicting the presence of knots in protein structures solely from their amino acid sequences. We tested the model's capabilities on 100 proteins whose structures had not yet been predicted by AF and found agreement with our local prediction in 92% cases. From the point of view of structural biology, we found that all potentially knotted proteins predicted by AF can be classified only into 17 families. This allows us to discover the presence of unknotted proteins in families with a highly conserved knot. We found only three new protein families: UCH, DUF4253, and DUF2254, that contain both knotted and unknotted proteins, and demonstrate that deletions within the knot core could potentially account for the observed unknotted (trivial) topology. Finally, we have shown that in the majority of knotted families (11 out of 15), the knotted topology is strictly conserved in functional proteins with very low sequence similarity. We have conclusively demonstrated that proteins AF predicts as unknotted are structurally accurate in their unknotted configurations. However, these proteins often represent nonfunctional fragments, lacking significant portions of the knot core (amino acid sequence).

Knotted artifacts in predicted 3D RNA structures

Unlike proteins, RNAs deposited in the Protein Data Bank do not contain topological knots. Recently, admittedly, the first trefoil knot and some lasso-type conformations have been found in experimental RNA structures, but these are still exceptional cases. Meanwhile, algorithms predicting 3D RNA models have happened to form knotted structures not so rarely. Interestingly, machine learning-based predictors seem to be more prone to generate knotted RNA folds than traditional methods. A similar situation is observed for the entanglements of structural elements. In this paper, we analyze all models submitted to the CASP15 competition in the 3D RNA structure prediction category. We show what types of topological knots and structure element entanglements appear in the submitted models and highlight what methods are behind the generation of such conformations. We also study the structural aspect of susceptibility to entanglement. We suggest that predictors take care of an evaluation of RNA models to avoid publishing structures with artifacts, such as unusual entanglements, that result from hallucinations of predictive algorithms.

Tying a true topological protein knot by cyclization

Knotted proteins are fascinating to biophysicists because of their robust ability to fold into intricately defined three-dimensional structures with complex and topologically knotted arrangements. Exploring the biophysical properties of the knotted proteins is of significant interest, as they could offer enhanced chemical, thermal, and mechanostabilities. A true mathematical knot requires a closed path; in contrast, knotted protein structures have open N- and C-termini. To address the question of how a truly knotted protein differs from the naturally occurring counterpart, we enzymatically cyclized a 31 knotted YibK protein from Haemophilus influenza (HiYibK) to investigate the impact of path closure on its structure-function relationship and folding stability. Through the use of a multitude of structural and biophysical tools, including X-ray crystallography, NMR spectroscopy, small angle X-ray scattering, differential scanning calorimetry, and isothermal calorimetry, we showed that the path closure minimally perturbs the native structure and ligand binding of HiYibK. Nevertheless, the cyclization did alter the folding stability and mechanism according to chemical and thermal unfolding analysis. These molecular insights contribute to our fundamental understanding of protein folding and knotting that could have implications in the protein design with higher stabilities.

Structure, dynamics, and stability of the smallest and most complex 71 protein knot

Proteins can spontaneously tie a variety of intricate topological knots through twisting and threading of the polypeptide chains. Recently developed artificial intelligence algorithms have predicted several new classes of topological knotted proteins, but the predictions remain to be authenticated experimentally. Here, we showed by X-ray crystallography and solution-state NMR spectroscopy that Q9PR55, an 89-residue protein from Ureaplasma urealyticum, possesses a novel 71 knotted topology that is accurately predicted by AlphaFold 2, except for the flexible N terminus. Q9PR55 is monomeric in solution, making it the smallest and most complex knotted protein known to date. In addition to its exceptional chemical stability against urea-induced unfolding, Q9PR55 is remarkably robust to resist the mechanical unfolding-coupled proteolysis by a bacterial proteasome, ClpXP. Our results suggest that the mechanical resistance against pulling-induced unfolding is determined by the complexity of the knotted topology rather than the size of the molecule.