⚛️ biophysics

The cytokine structural homologs IL-1β and IL-1Ra have distinct dynamical character that potentially influence their roles in allosteric regulation of the IL-1 receptor.

Despite sharing a nearly identical fold and average backbone flexibility, the cytokine IL-1β and its antagonist IL-1Ra exhibit distinct functional roles because the agonist's receptor-binding interface possesses anomalously high local mobility, demonstrating that the spatial distribution of conformational entropy, rather than its total amount, dictates allosteric regulation and biological outcome.

Original authors: Torres-Montalvo, G., Cole, T., Bishop, A. C., Lopes, R., Wand, A. J.

Published 2026-09-14

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CC BY 4.0

Original authors: Torres-Montalvo, G., Cole, T., Bishop, A. C., Lopes, R., Wand, A. 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 ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed.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

The immune system relies on a delicate balance of signals to decide when to fight an infection and when to stand down. Among the most important messengers in this system are proteins called cytokines, which act like cellular text messages telling other cells what to do. Two such proteins, interleukin-1 beta and interleukin-1 receptor antagonist, are structural twins. They are built from the same basic blueprint, shaped almost identically, and they both attach to the same receptor on the surface of a cell. Yet, despite their near-perfect resemblance, they send opposite commands. One triggers a powerful inflammatory response to fight invaders, while the other blocks that same receptor to calm the system down and prevent damage. For decades, scientists have known that these two proteins behave differently, but the reason for this divergence has remained hidden in the static snapshots provided by traditional structural biology. The question has been: if they look the same, how do they do such different things?

A team of researchers at Texas A&M University has now looked beyond the shape of these proteins to examine how they move. Using a technique called nuclear magnetic resonance, which allows scientists to watch atoms in motion, they measured the internal vibrations and wiggles of both proteins while they floated freely in solution. The study focused on the tiny, rapid movements of the protein's side chains, which are the chemical branches that stick out from the main backbone. These fast motions are not random noise; they represent a form of stored energy known as conformational entropy. The researchers found that while the two proteins possess the exact same total amount of this internal energy, they distribute it in completely different ways. The protein that triggers inflammation keeps its most active, jiggly parts right on the surface where it meets the receptor. In contrast, the protein that blocks inflammation keeps those same spots stiff and rigid, while its jiggly parts are tucked away elsewhere.

This discovery changes the understanding of how these biological switches work. The researchers showed that the inflammatory protein presents a surprisingly flexible and mobile surface to the cell's receptor. This mobility is not a flaw but a feature; it suggests that the protein is ready to change shape and rearrange itself to grab onto a second helper protein, which is required to start the signaling cascade. The blocking protein, however, presents a surface that is already stiff and pre-organized. It binds tightly to the receptor but lacks the flexibility needed to recruit that second helper, effectively freezing the signal before it can begin. The study measured the motion of hundreds of specific points on the proteins and found that the average amount of movement across the entire molecule was identical for both. The difference lay entirely in the location of that movement. The inflammatory protein concentrates its flexibility at the binding site, while the blocking protein concentrates its rigidity there.

The researchers were careful to rule out other possibilities. They confirmed that the proteins do not differ in their overall size or how they spin in solution, and they verified that the difference in motion was not an artifact of how the samples were prepared. By comparing two independently made batches of the inflammatory protein, they proved that their measurements were precise and reproducible. They also found that the proteins share a common evolutionary ancestor, suggesting that nature did not need to redesign the entire structure to create a new function. Instead, evolution simply shifted where the internal energy was kept. The inflammatory version kept the binding surface loose and ready to move, while the blocking version evolved to stiffen that same surface, turning a potential activator into a silent blocker.

This work highlights that the function of a protein is not determined solely by its shape, but also by the specific pattern of its internal motion. Two molecules can look identical in a photograph and still act as opposites because one is built to be flexible in the right place, while the other is built to be rigid. The findings suggest that the key to distinguishing between an activator and a blocker is not how much energy a protein has, but where it chooses to keep that energy. For the inflammatory protein, the binding surface is anomalously mobile, a trait that allows it to engage the full signaling machinery. For the blocking protein, the surface is pre-organized and stiff, allowing it to bind tightly without triggering the next step. This insight offers a new way to think about how proteins work and how they might be engineered or targeted by drugs. If a drug could be designed to stiffen the flexible surface of the inflammatory protein, it might stop the signal without blocking the receptor entirely, offering a new strategy for treating inflammatory diseases. The study confirms that the dynamic character of a protein is just as critical as its static structure in determining its biological role.

Technical Summary: Distinct Dynamical Character of IL-1β and IL-1Ra in Allosteric Regulation

Problem and Motivation
Interleukin-1β (IL-1β) and the interleukin-1 receptor antagonist (IL-1Ra) are structural homologs sharing the conserved β-trefoil fold. Despite engaging the same primary receptor (IL-1R1) with high affinity, they produce diametrically opposed biological outcomes: IL-1β acts as an agonist by recruiting the co-receptor IL-1RAcP to initiate inflammatory signaling, whereas IL-1Ra acts as a competitive antagonist, occupying the receptor without supporting co-receptor recruitment. While static crystal structures reveal subtle differences, they fail to explain the thermodynamic basis for this functional divergence. The authors posit that molecular recognition is governed not only by shape complementarity but also by the conformational states populated prior to binding and the entropy changes during complex formation. Specifically, the study seeks to determine if differences in residual conformational entropy—quantifiable via fast internal motions—underlie the distinct allosteric regulation of the IL-1 receptor.

Methodology
The study employed solution NMR spectroscopy to characterize the fast internal dynamics (ps-ns timescale) of both proteins.

Sample Preparation: Isotopically labeled wild-type IL-1β and wild-type IL-1Ra were prepared. Crucially, two independently prepared IL-1β samples were used to establish experimental precision directly, while IL-1Ra was analyzed as a single production sample. The formation of the C66-C122 disulfide bond in wild-type IL-1Ra was confirmed by chemical shift analysis.
Resonance Assignment: Extensive backbone and side-chain assignments were performed using triple resonance and total correlation spectra, achieving high coverage (>98% for non-proline amides and methyl groups).
Rotational Diffusion Analysis: To rigorously characterize macromolecular tumbling, extensive $^{15}$ N relaxation data ( $T_1$ , $T_2$ , heteronuclear NOE) were collected at multiple magnetic field strengths (500 MHz and 800 MHz). The authors addressed the challenge of the proteins' near-maximal triaxiality (structural rhombicity $\rho \approx 0.42-0.47$ ), which admits two near-degenerate axially symmetric solutions (oblate and prolate). Through exhaustive grid searches and jackknife resampling, they determined that while the symmetry axis is ambiguous, the effective correlation times ( $\tau_c$ ) are robust.
Order Parameter Determination:
- Backbone: Model-free squared generalized order parameters ( $O^2_{NH}$ ) were derived for backbone amides.
- Side Chains: Methyl-bearing side chain dynamics were quantified using cross-correlated relaxation rates ( $\eta$ and $\delta$ ) to extract $O^2_{axis}$ parameters.
Statistical Analysis: The precision of the data was validated by comparing the two independent IL-1β samples. Spatial distributions of dynamics were mapped onto the shared $\beta$ -trefoil fold using distance-weighted projections to alpha carbons.

Key Results

Global Dynamics are Indistinguishable: Both proteins exhibit nearly identical average internal motions. The mean backbone order parameters ( $O^2_{NH}$ ) are 0.805 for both. Similarly, the average methyl order parameters ( $\langle O^2_{axis} \rangle$ ) are statistically indistinguishable: $0.484 \pm 0.030$ for IL-1β and $0.486 \pm 0.033$ for IL-1Ra. This indicates that the total residual conformational entropy of the two proteins is essentially equivalent.
Spatial Distribution of Entropy is Divergent: While the amount of entropy is similar, its location differs significantly. The spatial distributions of methyl dynamics projected onto the shared fold are uncorrelated ( $R^2 = 0.021$ ). Regions that are rigid in one protein are often flexible in the other.
The Receptor Interface is the Critical Differentiator: The divergence in dynamics is concentrated at the receptor-binding surfaces.
- IL-1β (Agonist): The receptor footprint (Site A and Site B) is anomalously mobile. Methyl probes on the IL-1β footprint have a significantly lower order parameter ( $\langle O^2_{axis} \rangle = 0.325$ ) compared to the protein's global average. This mobility extends to the Site B interface required for IL-1RAcP recruitment.
- IL-1Ra (Antagonist): In contrast, the IL-1Ra receptor footprint is not distinguishable from its own global average ( $\langle O^2_{axis} \rangle = 0.495$ vs. $0.486$). The antagonist presents a pre-organized, rigid interface.
Thermodynamic Implications: IL-1Ra buries approximately 16% less surface area against the receptor than IL-1β yet binds with higher affinity. The authors calculate that IL-1Ra surrenders approximately 25 kJ mol $^{-1}$ less conformational entropy upon binding compared to IL-1β. The agonist's mobile interface incurs a larger entropic penalty upon ordering, while the antagonist's pre-organized interface minimizes this penalty.

Significance and Claims
The paper claims that conformational entropy can distinguish protein function even when static structures and average flexibility are conserved. The distinction between agonism and antagonism in this system does not reside in how much entropy a protein possesses, but where that entropy is kept.

Evolutionary Insight: The authors suggest that the ancestral state (IL-1β) presents a mobile binding surface, while the derived state (IL-1Ra) evolved to "stiffen" the interface. This pre-organization allows IL-1Ra to achieve high affinity and specificity without the entropic cost of ordering a flexible surface, effectively decoupling receptor occupancy from activation.
Mechanistic Distinction: The study demonstrates that the agonist's requirement for co-receptor recruitment is linked to the high mobility of its binding surfaces, which must rearrange to accommodate the ternary complex. The antagonist, by presenting a rigid surface, avoids this rearrangement, thereby preventing the allosteric transition required for signaling.
Therapeutic Relevance: The findings suggest that targeting the dynamic properties of the Site B interface (e.g., rigidifying the free-state conformation of IL-1β) could be a strategy to suppress co-receptor recruitment without blocking IL-1R1 binding, offering a mechanism distinct from simple competitive blockade.

In conclusion, the work establishes that the functional divergence of IL-1β and IL-1Ra is encoded in the spatial redistribution of conformational entropy, providing a dynamic explanation for their opposing biological roles that is invisible to static structural analysis.

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