- Intrinsically unstructured proteins
Intrinsically unstructured proteins, often referred to as naturally unfolded proteins or disordered proteins, are proteins characterized by lack of stable tertiary structure when the protein exists as an isolated polypeptide chain (a subunit) under physiological conditions in vitro. The discovery of intrinsically unfolded proteins challenged the traditional protein structure paradigm, which states that a specific well-defined structure was required for the correct function of a protein and that the structure defines the function of the protein. This is clearly not the case for intrinsically unfolded proteins that remain functional despite the lack of a well-defined structure. Such proteins, in some cases, can adopt a fixed three dimensional structure after binding to other macromolecules.
Biological role of intrinsic disorder
Many disordered proteins have the binding affinity with their receptors regulated by post-translational modification, thus it has been proposed that the flexibility of disordered proteins facilitates the different conformational requirements for binding the modifying enzymes as well as their receptors. Intrinsic disorder is particularly enriched in proteins implicated in cell signaling, transcription and chromatin remodeling functions.
Disordered regions are often found as flexible linkers (or loops) connecting two globular or transmembrane domains. Linker sequences vary greatly in length and amino acid sequence, but are similar in amino acid composition (rich in polar uncharged amino acids). Flexible linkers allow the connecting domains to freely twist and rotate through space to recruit their binding partners or for those binding partners to induce larger scale interdomain conformation changes.
Coupled folding and binding
Many unstructured proteins undergo transitions to more ordered states upon binding to their targets. The coupled folding and binding may be local, involving only a few interacting residues, or it might involve an entire protein domain. It was recently shown that the coupled folding and binding allows the burial of a large surface area that would be possible only for fully structured proteins if they were much larger. Moreover, certain disordered regions might serve as "molecular switches" in regulating certain biological function by switching to ordered conformation upon molecular recognition like small molecule-binding, DNA/RNA binding, ion interactions etc..
The ability of disordered proteins to bind, and thus to exert a function, shows that stability is not a required condition. Many short functional sites, for example Short Linear Motifs are over-represented in disordered proteins.
Sequence signatures of disorder
Intrinsically unstructured proteins are characterized by a low content of bulky hydrophobic amino acids and a high proportion of polar and charged amino acids. Thus disordered sequences cannot bury sufficient hydrophobic core to fold like stable globular proteins. In some cases, hydrophobic clusters in disordered sequences provide the clues for identifying the regions that undergo coupled folding and binding. Such signatures are the basis of the prediction methods below.
Many disordered proteins also reveal low complexity sequences, i.e. sequences with overrepresentation of a few residues. While low complexity sequences are a strong indication of disorder, the reverse is not necessarily true, that is, not all disordered proteins have low complexity sequences. Disordered proteins have a low content of predicted secondary structure.
Identification of intrinsically unstructured proteins
Intrinsically unfolded proteins, once purified, can be identified by various experimental methods. Folded proteins have a high density (partial specific volume of 0.72-0.74 mL/g) and commensurately small radius of gyration. Hence, unfolded proteins can be detected by methods that are sensitive to molecular size, density or hydrodynamic drag, such as size exclusion chromatography, analytical ultracentrifugation, Small angle X-ray scattering (SAXS), and measurements of the diffusion constant. Unfolded proteins are also characterized by their lack of secondary structure, as assessed by far-UV (170-250 nm) circular dichroism (esp. a pronounced minimum at ~200 nm) or infrared spectroscopy.
Unfolded proteins have exposed backbone peptide groups exposed to solvent, so that they are readily cleaved by proteases, undergo rapid hydrogen-deuterium exchange and exhibit a small dispersion (<1 ppm) in their 1H amide chemical shifts as measured by NMR. (Folded proteins typically show dispersions as large as 5 ppm for the amide protons.)
The primary method to obtain information on disordered regions of a protein is NMR spectroscopy. The lack of electron density in X-ray crystallographic studies may also be a sign of disorder.
Disorder and Disease
Intrinsically unstructured proteins have been implicated in a number of diseases. Aggregation of misfolded proteins is the cause of many synucleinopathies. The aggregation of the intrinsically unstructured protein α-Synuclein is thought to be responsible. The structural flexibility of this protein together with its susceptibility to modification in the cell leads to misfolded and aggregation.
Many key oncogenes have large intrinsically unstructured regions, for example p53 and BRCA1. These regions of the proteins are responsible for mediating many of their interactions.
De novo prediction of intrinsically unstructured proteins
Computational methods exploit the sequence signatures of disorder to predict whether a protein is disordered given its amino acid sequence. The table below, which was originally adapted from and has been recently updated, shows the main features of software for disorder prediction. Note that different software use different definitions of disorder.
Predictor What is predicted Based on Generates and uses multiple sequence alignment? PONDR All regions that are not rigid including random coils, partially unstructured regions, and molten globules Local aa composition, flexibility, hydropathy, etc No GlobPlot Regions with high propensity for globularity on the Russell/Linding scale (propensities for secondary structures and random coils) Russell/Linding scale of disorder No DisEMBL LOOPS (regions devoid of regular secondary structure); HOT LOOPS (highly mobile loops); REMARK465 (regions lacking electron density in crystal structure) Neural networks trained on X-ray structure data No SEG Low-complexity segments that is, “simple sequences” or “compositionally biased regions”. Locally optimized low-complexity segments are produced at defined levels of stringency and then refined according to the equations of Wootton and Federhen No Disopred2 Regions devoid of ordered regular secondary structure Cascaded support vector machine classifiers trained on PSI-BLAST profiles Yes OnD-CRF The transition between structurally ordered and mobile or disordered amino acids intervals under native conditions. OnD-CRF applies Conditional Random Fields, CRFs, which rely on features generated from the amino acid sequence and from secondary structure prediction. No NORSp Regions with No Ordered Regular Secondary Structure (NORS). Most, but not all, are highly flexible. Secondary structure and solvent accessibility Yes FoldIndex Regions that have a low hydrophobicity and high net charge (either loops or unstructured regions) Charge/hydrophaty analyzed locally using a sliding window No Charge/hydropathy method. Fully unstructured domains (random coils) Global sequence composition No HCA (Hydrophobic Cluster Analysis) Hydrophobic clusters, which tend to form secondary structure elements Helical visualization of amino acid sequence No PreLink Regions that are expected to be unstructured in all conditions, regardless of the presence of a binding partner Compositional bias and low hydrophobic cluster content. No IUPred Regions that lack a well-defined 3D-structure under native conditions Energy resulting from inter-residue interactions, estimated from local amino acid composition No RONN Regions that lack a well-defined 3D structure under native conditions Bio-basis function neural network trained on disordered proteins No MD (Meta-Disorder predictor) Regions of different "types"; for example, unstructured loops and regions containing few stable intra-chain contacts A neural-network based meta-predictor that uses different sources of information predominantly obtained from orthogonal approaches Yes GeneSilico Metadisorder Regions that lack a well-defined 3D structure under native conditions (REMARK-465) Meta method, which uses other disorder predictors (like RONN, IUPred, POODLE, and many more). Based on them the consensus is calculated according method accuracy (optimized using ANN, filtering and other techniques). Currently the best available method (first 2 places in last CASP experiment (blind test)) Yes IUPforest-L Long disordered regions in a set of proteins Moreau-Broto auto-correlation function of amino acid indices (AAIs) No MFDp  Different types of disorder including random coils, unstructured regions, molten globules, and REMARK-465-based regions. An ensemble of 3 SVMs specialized for the prediction of short, long and generic disordered regions, which combines three complementary disorder predictors, sequence, sequence profiles, predicted secondary structure, solvent accessibility, backbone dihedral torsion angles, residue flexibility and B-factors. MFDp (unofficially) secured 3rd place in last CASP experiment) Yes
Since the methods above use different definitions of disorder and they were trained on different datasets, it is difficult to estimate their relative accuracy, but disorder prediction category is a part of biannual CASP experiment that is designed to test methods according accuracy in finding regions with missing 3D structure.
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