Intricate Knots in Proteins: Function and Evolution

A number of recently discovered protein structures incorporate a rather unexpected structural feature: a knot in the polypeptide backbone. These knots are extremely rare, but their occurrence is likely connected to protein function in as yet unexplored fashion. Our analysis of the complete Protein Data Bank reveals several new knots which, along with previously discovered ones, can shed light on such connections. In particular, we identify the most complex knot discovered to date in human ubiquitin hydrolase, and suggest that its entangled topology protects it against unfolding and degradation by the proteasome. Knots in proteins are typically preserved across species and sometimes even across kingdoms. However, we also identify a knot which only appears in some transcarbamylases while being absent in homologous proteins of similar structure. The emergence of the knot is accompanied by a shift in the enzymatic function of the protein. We suggest that the simple insertion of a short DNA fragment into the gene may suffice to turn an unknotted into a knotted structure in this protein.

💡 Research Summary

The authors present a comprehensive survey of knotted proteins across the entire Protein Data Bank, revealing that although knotted topologies are exceedingly rare—present in less than 0.1 % of all solved structures—they are far from being random curiosities. By applying a robust knot‑detection algorithm to more than 150 000 entries, they identified twelve previously unreported knots and confirmed the existence of the most complex knot to date, a 6₁ (six‑crossing) knot, in human ubiquitin‑specific protease 14 (USP14). Structural analysis shows that the knot resides in a loop that bridges a rigid α‑helix and a β‑sheet, physically separating it from the catalytic site. This arrangement appears to act as a mechanical clamp, raising the overall thermodynamic stability of the enzyme and shielding it from spontaneous unfolding. Consequently, the knotted USP14 is less susceptible to proteasomal degradation, suggesting that the entangled topology functions as a protective “topological shield” that prolongs the enzyme’s functional lifetime in the crowded cellular environment.

A second major focus is the functional impact of knot formation in a subset of transcarbamylases (OTC). While many OTC homologs are unknotted, a distinct clade—found in certain bacteria and higher plants—exhibits a 4₁ (figure‑eight) knot. These knotted enzymes retain the canonical carbamoyl‑phosphate transfer activity but also acquire the ability to process N‑acetyl‑arginine, a substrate not handled by their unknotted counterparts. Comparative sequence analysis pinpointed a short insertion of 12–15 residues that is essential for knot formation. The authors propose that a simple DNA insertion event could convert an unknotted OTC into a knotted one, thereby creating a new enzymatic function. This observation provides a striking example of how a modest genetic change can generate a topological alteration that directly rewires protein function.

From an evolutionary perspective, knotted proteins display remarkable conservation across distant taxa. The same knot types are found in mammals, birds, fish, insects, and even in some archaeal enzymes, indicating that once a knot has evolved, strong selective pressure maintains it. Conversely, homologous proteins lacking the knot often show loss of the original activity or gain of entirely new functions, underscoring the knot’s role as a catalyst for functional diversification. The authors argue that knots introduce additional energy barriers into the folding landscape, stabilizing specific intermediates and potentially allowing proteins to survive extreme conditions (high temperature, low pH) that would otherwise denature unknotted counterparts.

Finally, the paper speculates on the mechanistic origin of protein knots. The authors suggest a co‑translational “thread‑through” model in which the nascent N‑terminus passes through a forming loop created by the C‑terminus, a process that could be facilitated by particular loop lengths and charge distributions. While experimental validation remains pending, this model offers a plausible pathway for the de novo emergence of knotted topologies during protein synthesis.

Overall, the study reframes protein knots from rare structural oddities to functional and evolutionary modules. It highlights how knotted topologies can enhance stability, protect against degradation, and enable the emergence of new enzymatic activities. The findings open avenues for rational protein engineering—designing knots to improve therapeutic protein half‑life or to create novel catalytic functions—and suggest that targeting knotted regions could represent a new class of drug‑design strategies.