Intrinsically disordered proteins (IDPs) fold into ordered structures as they bind their physiological partners. A 2012 topology-based simulation study of the synergistic folding of two IDPs — the nuclear coactivator binding domain (NCBD) of the CREB-binding protein (CBP) and the activation domain ACTR of the coactivator NCOA3 — left open a bundle of mechanistic questions: the kinetic roles of non-native contacts and long-range electrostatics that its calibrated Gō-like model omitted, the attainment of quantitative, physics-based (atomistic) rate predictions for coupled binding and folding, and the general physical mechanism by which IDPs fold so efficiently upon binding, often approaching diffusion-limited association. The electrostatic and residual-structure components have since been substantially resolved experimentally and computationally, and atomistic simulations now deliver mechanistic descriptions for several systems. What remains open is a validated quantitative account: predicting, from atomistic physics-based models with explicit electrostatics and non-native interactions, the measured association and dissociation kinetics of coupled binding–folding systems such as NCBD/ACTR (including their salt, mutation, and residual-helicity dependence and the structure and lifetime of encounter complexes), together with a quantitative mechanistic understanding of when and why IDP association reaches the diffusion limit.

Background

Intrinsically disordered proteins (IDPs) are proteins that, unlike globular proteins, do not adopt a single stable tertiary fold in isolation under physiological conditions, instead populating a heterogeneous ensemble of rapidly interconverting conformations; they are abundant in cellular signaling and regulation. A common functional motif of IDPs is coupled binding and folding: a segment that is disordered when free folds into a defined structure upon binding a partner. Two idealized limiting mechanisms are usually distinguished. In conformational selection, the partner binds and stabilizes a pre-existing, folded-like conformation that the free IDP already samples transiently; in induced folding (induced fit), binding precedes folding, with intermolecular contacts driving the disorder-to-order transition. Real systems often mix the two.

A paradigm for the most extreme version of this phenomenon — synergistic (mutual) folding, in which both partners are disordered and stabilize each other's folded state — is the complex between NCBD (the nuclear coactivator binding domain of the CREB-binding protein, CBP, a transcriptional coactivator) and ACTR (the disordered activation domain of the p160 steroid receptor coactivator NCOA3). Free NCBD is a molten globule, meaning a compact, partially folded state with pronounced native-like helicity but no rigid tertiary packing, while free ACTR is largely unstructured; together they fold into a single well-packed three-helix-bundle complex.

Ganguly, Zhang, and Chen (2012) studied this pair with a topology-based (Gō-like) model — a coarse-grained simulation in which each residue is one bead and favorable interactions are assigned essentially only between residue pairs that are in native contact in the known complex structure, so that the model encodes the folded topology by construction. They calibrated this model to reproduce the residual helicity of unbound ACTR and the experimental binding affinity, and concluded that recognition follows an extended conformational selection mechanism for NCBD (multiple stages of selection and induced folding) with ACTR folding largely induced by binding, initiated by a mini folded core involving the C-terminal helices of both proteins. The authors explicitly acknowledged the model's limits: native-centric potentials cannot represent non-native residual structure or transient, nonspecific encounter complexes (the short-lived, partially formed assemblies on the way to the specific complex), long-range electrostatic interactions are absent, and the approach cannot deliver quantitative, atomically resolved kinetic predictions. They also posed the broader mechanistic puzzle that their results underscored: by what physical mechanisms do IDPs fold so fast upon binding — often with association rate constants approaching the diffusion limit — when folding itself could impose a kinetic bottleneck; candidate explanations included the low contact order (few long-range contacts, hence fast local folding) of binding motifs and long-range electrostatic steering (acceleration of encounter by charge–charge attraction).

Subsequent work transformed parts of this picture. Stopped-flow kinetics (rapid mixing of reactants to resolve sub-second reaction time courses) showed that NCBD/ACTR association (about 3×107 M−1s−13\times10^{7}\,\mathrm{M^{-1}s^{-1}} at 277 K) is governed by electrostatics via a conserved buried intermolecular salt bridge, with slow conformational steps following; Φ\Phi-value analysis (protein-engineering analysis of which native contacts form in the kinetic transition state) found a largely disordered-like transition state; single-molecule fluorescence resolved a metastable, electrostatically favored encounter complex that converts to the bound state within tens of microseconds; stabilizing ACTR's residual helices was shown to accelerate binding by making nonspecific collisions evolve more productively rather than by seeding native contacts; and single-molecule work on another IDP system demonstrated that genuinely diffusion-limited association can be achieved through non-native electrostatic stabilization of an unusually long-lived transient complex. Atomistic, explicit-solvent simulations have now sampled complete coupled binding–folding landscapes for several systems — for example measles virus NTAIL\mathrm{N_{TAIL}}/XD (the disordered C-terminal tail of the measles-virus nucleoprotein and its folded partner domain XD) and c-Myb/KIX (the disordered transactivation region of the transcription factor c-Myb and the KIX interaction domain of CBP) — revealing induced-folding-dominated mechanisms and mixed pathways. What has not been achieved is the quantitative tier: a physics-based, atomistic model that predicts the measured rate constants of a coupled binding–folding reaction — and their dependence on ionic strength, mutations, and residual-structure stabilization — from sequence and structure alone, together with a general quantitative theory of the folding-upon-binding speed limit.

Problem Statement

Establish a validated, quantitative, physics-based account of coupled binding and folding in synergistic IDP–IDP recognition, with the NCBD/ACTR pair as the reference system, in two linked parts that together close the acknowledged limitations of the 2012 topology-based analysis:

  1. Quantitative kinetic prediction. Construct atomistic (explicit-solvent, explicit-electrostatics, non-native interactions included) simulation models — with whatever enhanced-sampling or kinetic-reconstruction machinery is required — that predict, from sequence and the structures of the partners, the experimentally measured association and dissociation rate constants of the NCBD/ACTR reaction, validated against the available experimental kinetic data for this system, such as the association rate of order 107 M−1s−110^{7}\,\mathrm{M^{-1}s^{-1}} (about 3×107 M−1s−13\times10^{7}\,\mathrm{M^{-1}s^{-1}} at 277 K) with its ionic-strength dependence, the effect of mutating the conserved intermolecular salt bridge, the acceleration by stabilized ACTR residual helices, the cis/trans proline-linked kinetic heterogeneity, and the structure, lifetime, and productive-versus-unproductive evolution of encounter complexes en route to the folded complex. These experimental observations serve as validation targets rather than a mandatory completion checklist.

  2. The general speed-limit mechanism. Determine quantitatively the physical mechanisms by which IDPs fold so efficiently upon binding — often with association approaching the diffusion limit — i.e., establish when IDP association is diffusion-limited and what sets the folding-upon-binding speed, dissecting the relative contributions of long-range electrostatic steering, non-native transient-complex stabilization, residual structure, and the low contact order of binding motifs, so that the qualitative mechanisms identified since 2012 (electrostatic steering, long-lived electrostatically stabilized transient complexes, residual-helicity-promoted productive encounters) are connected to rates that experiments can test.

The problem is solved when both linked parts are satisfied: part 1 by an account that reproduces the measured association and dissociation kinetics of the NCBD/ACTR system within experimental uncertainty, and part 2 by a quantitative mechanistic account of when IDP association is diffusion-limited and what sets the folding-upon-binding speed, dissecting the relative contributions of long-range electrostatic steering, non-native transient-complex stabilization, residual structure, and the low contact order of binding motifs, demonstrated on at least one additional coupled binding–folding IDP system beyond the NCBD/ACTR reference system. Together they deliver quantitative, physics-based (atomistic) kinetic predictions that calibrated native-centric models cannot provide and complement, rather than replace, them as sources of mechanistic evidence for IDP recognition.

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