Molecular communication in biology is mediated by protein interactions. According to the current paradigm, the specificity and affinity required for these interactions are encoded in the precise complementarity of binding interfaces. Even proteins that are disordered under physiological conditions or that contain large unstructured regions commonly interact with well-structured binding sites on other biomolecules. Here we demonstrate the existence of an unexpected interaction mechanism: the two intrinsically disordered human proteins histone H1 and its nuclear chaperone prothymosin-α associate in a complex with picomolar affinity, but fully retain their structural disorder, long-range flexibility and highly dynamic character. On the basis of closely integrated experiments and molecular simulations, we show that the interaction can be explained by the large opposite net charge of the two proteins, without requiring defined binding sites or interactions between specific individual residues. Proteome-wide sequence analysis suggests that this interaction mechanism may be abundant in eukaryotes.

The dimensions of unfolded and intrinsically disordered proteins are highly dependent on their amino acid composition and solution conditions, especially salt and denaturant concentration. However, the quantitative implications of this behavior have remained unclear, largely because the effective theta-state, the central reference point for the underlying polymer collapse transition, has eluded experimental determination. Here, we used single-molecule fluorescence spectroscopy and two-focus correlation spectroscopy to determine the theta points for six different proteins. While the scaling exponents of all proteins converge to 0.62 AE 0.03 at high denaturant concentrations, as expected for a polymer in good solvent, the scaling regime in water strongly depends on sequence composition. The resulting average scaling exponent of 0.46 AE 0.05 for the four foldable protein sequences in our study suggests that the aqueous cellular milieu is close to effective theta conditions for unfolded proteins. In contrast, two intrinsically disordered proteins do not reach the Θ-point under any of our solvent conditions, which may reflect the optimization of their expanded state for the interactions with cellular partners. Sequence analyses based on our results imply that foldable sequences with more compact unfolded states are a more recent result of protein evolution.protein folding | single-molecule FRET | coil-globule transition | polymer theory I t has become increasingly clear that the structure and dynamics of unfolded proteins are essential for understanding protein folding (1-3) and the functional properties of intrinsically disordered proteins (IDPs) (4-6). Theoretical concepts from polymer physics (7-9) have frequently been used to describe the properties of unfolded polypeptide chains (4, 10, 11) with the goal to establish the link between protein folding and collapse (12-15). However, the methodology to test many of these concepts experimentally has only become available rather recently (2,16,17). A considerable body of experimental and theoretical work suggests that the dimensions of unfolded proteins depend on parameters such as amino acid composition (4), temperature (18), and solvent quality (3,10,15,19). The continuous collapse of polymers has been treated exhaustively by a number of theories (20-24) based on general principles that relate the dimensions and the length of a chain to its free energy. However, a prerequisite for the quantitative application of these theories and their comparison to experimental results is that the dimensions of the Θ-state are known, which serves as an essential reference state. At the Θ-point*, chain-chain and chain-solvent interactions balance such that the polymer is at a critical point, at which the thermodynamic phase boundaries disappear. As a result, the polypeptide chain obeys the same length scaling as an ideal chain without excluded volume and intrachain interactions. However, the Θ-conditions for protein chains are unknown. Besides its importance for obtaining the corre...

There has been a long-standing controversy regarding the effect of chemical denaturants on the dimensions of unfolded and intrinsically disordered proteins: A wide range of experimental techniques suggest that polypeptide chains expand with increasing denaturant concentration, but several studies using small-angle X-ray scattering (SAXS) reported no such increase of the radius of gyration (Rg). This inconsistency challenges our current understanding of the mechanism of chemical denaturants, which are widely employed to investigate protein folding and stability. Here, we use a combination of single-molecule Förster resonance energy transfer (FRET), SAXS, dynamic light scattering (DLS), and two-focus fluorescence correlation spectroscopy (2f-FCS) to characterize the denaturant dependence of the unfolded state of the spectrin domain R17 and the intrinsically disordered protein ACTR in two different denaturants. Standard analysis of the primary data clearly indicates an expansion of the unfolded state with increasing denaturant concentration irrespective of the protein, denaturant, or experimental method used. This is the first case in which SAXS and FRET have yielded even qualitatively consistent results regarding expansion in denaturant when applied to the same proteins. To more directly illustrate this self-consistency, we have used both SAXS and FRET data in a Bayesian procedure to refine structural ensembles representative of the observed unfolded state. This analysis demonstrates that both of these experimental probes are compatible with a common ensemble of protein configurations for each denaturant concentration. Furthermore, the resulting ensembles reproduce the trend of increasing hydrodynamic radius with denaturant concentration obtained by 2f-FCS and DLS. We were thus able to reconcile the results from all four experimental techniques quantitatively, to obtain a comprehensive structural picture of denaturant-induced unfolded state expansion, and to identify the most likely sources of earlier discrepancies.

Although protein-folding studies began several decades ago, it is only recently that the tools to analyze protein folding at the single-molecule level have been developed. Advances in single-molecule fluorescence and force spectroscopy techniques allow investigation of the folding and dynamics of single protein molecules, both at equilibrium and as they fold and unfold. The experiments are far from simple, however, both in execution and in interpretation of the results. In this review, we discuss some of the highlights of the work so far and concentrate on cases where comparisons with the classical experiments can be made. We conclude that, although there have been relatively few startling insights from single-molecule studies, the rapid progress that has been made suggests that these experiments have significant potential to advance our understanding of protein folding. In particular, new techniques offer the possibility to explore regions of the energy landscape that are inaccessible to classical ensemble measurements and, perhaps, to observe rare events undetectable by other means.

Energy landscape theory is a powerful tool for understanding the structure and dynamics of complex molecular systems, in particular biological macromolecules1. The primary sequence of a protein defines its free energy landscape, and thus determines the folding pathway and the rate constants of folding and unfolding, as well as its native structure. Theory has shown that roughness in the energy landscape will lead to slower folding1, but derivation of detailed experimental descriptions of this landscape is challenging. Simple folding models2,3 show that folding is significantly influenced by chain entropy; proteins where the contacts are local fold fast, due to the low entropy cost of forming stabilising, native contacts during folding4,5. For some protein families, stability is also a determinant of folding rate constants6. Where these simple metrics fail to predict folding behaviour it is probable that there are features in the energy landscape that are unusual. Such general observations cannot explain the folding behaviour of the R15, R16 and R17 domains of α-spectrin. R15 folds ~3000 times faster than its homologues, although they have similar structures, stabilities and, as far as can be determined, transition state stabilities7-10. Here we show that landscape roughness (internal friction) is responsible for the slower folding and unfolding of R16 and R17. We use chimeric domains to demonstrate that this internal friction is a property of the core, and suggest that frustration in the landscape of the slow folding spectrin domains may be due to mis-docking of the long helices during folding. Although theoretical studies have suggested that rugged landscapes will result in slower folding, this is the first time that such a phenomenon has been shown experimentally to directly influence the folding kinetics of a “normal” protein with a significant energy barrier – one which folds on a relatively slow ms-s timescale.

A large range of debilitating medical conditions1 are linked to protein misfolding, which may compete with productive folding particularly in proteins containing multiple domains2. With 75% of the eukaryotic proteome consisting of multidomain proteins, how is inter-domain misfolding avoided? It has been proposed that maintaining low sequence identity between covalently linked domains is a mechanism to avoid misfolding3. Here we use single-molecule Förster Resonance Energy Transfer (FRET) experiments4,5 to detect and quantify rare misfolding events in tandem Ig domains from the I-band of titin under native conditions. About 5.5% of molecules with identical domains misfold during refolding in vitro and form a surprisingly stable state with an unfolding half time of several days. Tandem arrays of immunoglobulin-like (Ig-like) domains in humans exhibit significantly lower sequence identity between neighbouring domains than between non-adjacent domains3. In particular, the sequence identity of neighbouring domains has been found to be preferentially below 40%3. Interestingly we observe no misfolding for a tandem of naturally neighbouring domains with low sequence identity (24%), whereas misfolding occurs between domains which are 42% identical. Coarse-grained molecular simulations predict the formation of domain-swapped structures, which are in excellent agreement with the observed transfer efficiency of the misfolded species. We infer that the interactions underlying misfolding are very specific and result in a sequence-specific domain swapping mechanism. Diversifying the sequence between neighbouring domains appears to be a successful evolutionary strategy to avoid misfolding in multidomain proteins.

Theory, simulations and experimental results have suggested an important role of internal friction in the kinetics of protein folding. Recent experiments on spectrin domains provided the first evidence for a pronounced contribution of internal friction in proteins that fold on the millisecond timescale. However, it has remained unclear how this contribution is distributed along the reaction and what influence it has on the folding dynamics. Here we use a combination of single-molecule Förster resonance energy transfer, nanosecond fluorescence correlation spectroscopy, microfluidic mixing and denaturant- and viscosity-dependent protein-folding kinetics to probe internal friction in the unfolded state and at the early and late transition states of slow- and fast-folding spectrin domains. We find that the internal friction affecting the folding rates of spectrin domains is highly localized to the early transition state, suggesting an important role of rather specific interactions in the rate-limiting conformational changes.

The folding mechanism of many proteins involves the population of partially organized structures en route to the native state. Identification and characterization of these intermediates is particularly difficult, as they are often only transiently populated and may play different mechanistic roles, being either on-pathway productive species or off-pathway kinetic traps. Following different spectroscopic probes, and employing state-of-the-art kinetic analysis, we present evidence that the folding mechanism of the thermostable cytochrome c 552 from Hydrogenobacter thermophilus does involve the presence of an elusive, yet compact, on-pathway intermediate. Characterization of the folding mechanism of this cytochrome c is particularly interesting for the purpose of comparative folding studies, because H. thermophilus cytochrome c 552 shares high sequence identity and structural homology with its homologue from the mesophilic bacterium Pseudomonas aeruginosa cytochrome c 551 , which refolds through a broad energy barrier without the accumulation of intermediates. Analysis of the folding kinetics and correlation with the three-dimensional structure add new evidence for the validity of a consensus folding mechanism in the cytochrome c family.