⚛️ biophysics

The Rossmann2x2 Fold Attains its Native Structure Via a Defined Pathway of Sequential and Cooperative Folding Units

This single-molecule force spectroscopy study reveals that the Rossmann2x2 fold protein (Ross) reaches its native structure through a strict, reversible pathway where discrete cooperative folding units (foldons) assemble sequentially, starting with an autonomously folding primary unit that guides the subsequent organization of dependent secondary units.

Original authors: Sosa, R., Kim, J., Tong, A. B., Krishnamurthy, S., Volz, C., Hess, B., Pineda, D., Bustamante, C. J.

Published 2026-09-18

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CC BY 4.0

Original authors: Sosa, R., Kim, J., Tong, A. B., Krishnamurthy, S., Volz, C., Hess, B., Pineda, D., Bustamante, C. J.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Proteins are the workhorses of life, tiny molecular machines that build structures, catalyze reactions, and regulate the chemistry inside every living cell. To function, these long chains of amino acids must twist and fold into precise, three-dimensional shapes. For decades, scientists have known that a protein's final shape is determined entirely by its sequence of amino acids, a fact established in the 1960s. However, the journey a protein takes to get from a loose, tangled string to its final, functional form has remained a subject of intense debate. One school of thought suggests that proteins fold by randomly exploring countless possibilities, gradually finding their way down a smooth energy slope until they settle into their correct shape. Another view proposes that proteins follow a strict, step-by-step assembly line, where specific sections fold first and then guide the rest of the molecule into place. Resolving this question is crucial because when proteins fail to fold correctly, they can clump together and cause devastating diseases like Alzheimer's and Parkinson's.

A team of researchers has now captured the folding process of a specific protein in real time, revealing that it follows a rigid, ordered path rather than a random walk. The subject of their study is a protein called Ross, a representative of a very common structural family known as the Rossmann fold, which appears in more than 20 percent of all known proteins. Using a technique called optical tweezers, which employs focused beams of light to hold and stretch individual molecules, the scientists were able to pull on a single Ross protein and watch it unfold and refold thousands of times. They discovered that the protein does not collapse into its shape all at once, nor does it wander aimlessly. Instead, it assembles through a series of distinct, cooperative steps, like a set of nested blocks that must be placed in a specific order.

The researchers observed that the protein passes through two stable intermediate states during its journey to the final shape. By introducing small, flexible loops of amino acids at different points along the protein's chain, they were able to map exactly which parts of the molecule were folding at each stage. They found that the first section to fold is a specific group of five structural elements that can organize themselves independently. Once this initial unit is in place, it acts as a foundation, allowing a second group of elements to attach and stabilize. Only after these two groups are locked together can the final piece of the puzzle snap into place. This hierarchy means that the later parts of the protein are unstable and cannot fold on their own; they require the presence of the earlier, already-folded sections to hold them together.

To ensure that the stretching force used in the experiment was not forcing the protein into an unnatural path, the team developed a new method called rapid force quench. This technique involves stretching the protein to unfold it and then suddenly dropping the tension to near zero for a fraction of a second, allowing the molecule to relax and begin folding without any external pull. Even under these gentle, force-free conditions, the protein consistently began by forming that same initial group of five elements. This confirmed that the observed order is not an artifact of the experiment but the protein's natural, preferred route to its final shape. The study also identified a hidden, smaller unit within that first group that forms even earlier, suggesting the assembly process is even more granular than the initial observations showed.

The findings support a model where proteins are built from modular units called foldons, which assemble in a strict hierarchy. The first unit, or primary foldon, folds autonomously because its internal structure is strong enough to stand alone. The subsequent units are secondary; they are thermodynamically unstable unless they interact with the primary unit or other previously folded sections. This creates a one-way street for folding: the protein must build from the bottom up, and it unfolds in the exact reverse order. The researchers ruled out the idea that the protein could start folding at any random point along its chain or that it could take multiple different paths to reach the same destination. Instead, the data points to a single, defined pathway dictated by the energetic coupling between these discrete folding units.

This work provides a clear, concrete example of how a complex biological machine assembles itself with precision. By showing that the folding of a common protein class follows a predictable, sequential rule, the study offers a potential key to understanding the broader principles of protein folding. If this hierarchical, modular logic applies to other proteins, it could eventually allow scientists to predict how any natural protein will fold simply by looking at its amino acid sequence, turning a decades-old mystery into a solvable engineering problem. The researchers suggest that nature may have evolved these modular folding units to simplify the process, ensuring that proteins reach their functional shapes quickly and reliably, avoiding the dangerous traps of misfolding that lead to disease.

Technical Summary: The Rossmann2x2 Fold Attains its Native Structure Via a Defined Pathway of Sequential and Cooperative Folding Units

Problem Statement
While the prediction of static protein structures has been revolutionized by AI and machine learning (e.g., AlphaFold), the fundamental mechanism of how proteins fold into their native states remains a subject of debate. Two primary models exist: the energy landscape theory, which posits that proteins fold via a funnel-like surface with multiple possible pathways and nucleation sites; and the foldon model, which suggests proteins fold via a single, defined pathway involving the sequential assembly of discrete, cooperative units (foldons). Previous bulk experiments (e.g., HDX-MS) support the foldon model but cannot resolve individual folding trajectories in real time. Conversely, previous single-molecule force spectroscopy studies have identified discrete intermediates but lacked the resolution to definitively annotate their structural identity. This study addresses the "extended protein folding problem" by determining whether a conserved Rossmann2x2 fold follows a unique, hierarchical pathway and by identifying the specific structural units driving this process.

Methodology
The authors utilized a high-resolution single-molecule force spectroscopy approach using optical tweezers to study "Ross," a de novo-designed protein (PDB 2LV8) representing the Rossmann2x2 fold. The experimental strategy involved:

Single-Molecule Manipulation: Ross was tethered between two beads via dsDNA handles. The protein was subjected to repeated pulling/relaxing cycles and passive mode (equilibrium) experiments to observe unfolding and refolding transitions.
Structural Annotation via Gly-Loop Insertion: To identify the structural composition of folding intermediates, the authors generated variants of Ross with inserted 18-glycine (Gly-loop) segments between adjacent secondary structures. The insertion of an unstructured loop increases the contour length ( $L_c$ ) of a transition by a predictable amount (~6.3 nm) if that specific region unfolds during the transition.
In-situ Fragment Isolation: Using a protease-cleavable loop and engineered cysteines, specific regions of the protein were isolated in situ within the optical trap. This allowed the authors to test the autonomous folding capability of specific foldon combinations (e.g., the first foldon alone vs. the first and second foldons together).
Rapid Force Quench (RFQ): To rule out force-induced bias, a RFQ method was employed. The protein was unfolded, and the force was rapidly quenched to near zero pN for a short duration (300 µsec) before being re-applied. This captured folding intermediates formed under near-physiological, zero-tension conditions.
Computational Validation: Molecular dynamics (MD) simulations and Rosetta relaxations were used to validate the stability of proposed foldons and the effect of Gly-loop insertions.

Key Results

Discrete and Reversible Intermediates: Analysis of 823 transitions revealed two well-defined intermediates, I1 and I2, located at approximately 47% and 23% of the total contour length, respectively. These intermediates were observed in both pulling/relaxing and passive mode experiments, confirming a reversible, cooperative folding pathway.
Identification of Foldons: The Gly-loop insertion experiments mapped the structural identity of the intermediates:
- First Foldon (Primary): The transition from Unfolded (U) to I2 involves the cooperative organization of $\beta1\alpha2\beta3\alpha4\beta5$ . This unit is termed the "primary foldon."
- Second Foldon: The transition from I2 to I1 involves the addition of $\alpha6\beta7$ .
- Third Foldon: The final step from I1 to the Folded (F) state involves $\alpha8$ .
Hierarchical Autonomy: In-situ isolation experiments demonstrated a strict hierarchy of folding autonomy:
- The primary foldon ( $\beta1\alpha2\beta3\alpha4\beta5$ ) folds autonomously and stably, even when isolated from the rest of the protein.
- The second foldon ( $\alpha6\beta7$ ) and third foldon ( $\alpha8$ ) are thermodynamically unstable and do not fold when isolated; they require the prior organization of the primary foldon (and in the case of the third, the first two) to stabilize.
- This establishes a "primary" foldon that initiates folding, followed by "secondary" foldons whose stability is energetically coupled to previously organized units.
Zero-Force Validation: RFQ experiments confirmed that the folding pathway observed under force is not an artifact of tension. At near-zero force, the protein first forms the primary foldon (I2), then transitions to I1, and finally reaches the native state, mirroring the pathway observed under force.
Hidden Intermediate: MD simulations and further analysis suggested the existence of a transient, short-lived intermediate ( $\beta1\alpha2\beta3$ ) within the primary foldon, which acts as the initial nucleation core.

Significance and Claims
The paper claims that the Rossmann2x2 fold follows a single, unique, and hierarchical folding pathway dictated by the intrinsic stability and energetic coupling of discrete folding units (foldons). The findings support the foldon model over the "diffusion on a funnel" model for this specific topology. The authors propose that the folding mechanism is simplified by a modular architecture where a primary foldon nucleates the process, and subsequent foldons assemble in a strict order dependent on their interactions with the primary unit.

The authors suggest that if this foldon hierarchy is a general principle, the folding pathway of natural proteins could potentially be predicted from their sequence by identifying these autonomous folding units and their relative stabilities. They conclude that proteins may be better described as oligomers of foldons rather than simple polymers of amino acids, offering a potential simplification for understanding the "extended protein folding problem." The study emphasizes that this conclusion is specific to the Ross protein and the Rossmann2x2 fold, with future work proposed to validate these rules across other conserved folds.

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