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Intrinsically disordered proteins play diverse roles in cell signaling

Open access
Bondos, Sarah E., Dunker, A. Keith, Uversky, Vladimir N.·2022·Cell Communication and Signaling

Abstract

Signaling pathways allow cells to detect and respond to a wide variety of chemical (e.g. Ca 2+ or chemokine proteins) and physical stimuli (e.g., sheer stress, light). Together, these pathways form an extensive communication network that regulates basic cell activities and coordinates the function of multiple cells or tissues. The process of cell signaling imposes many demands on the proteins that comprise these pathways, including the abilities to form active and inactive states, and to engage in multiple protein interactions. Furthermore, successful signaling often requires amplifying the signal, regulating or tuning the response to the signal, combining information sourced from multiple pathways, all while ensuring fidelity of the process. This sensitivity, adaptability, and tunability are possible, in part, due to the inclusion of intrinsically disordered regions in many proteins involved in cell signaling. The goal of this collection is to highlight the many roles of intrinsic disorder in cell signaling. Following an overview of resources that can be used to study intrinsically disordered proteins, this review highlights the critical role of intrinsically disordered proteins for signaling in widely diverse organisms (animals, plants, bacteria, fungi), in every category of cell signaling pathway (autocrine, juxtacrine, intracrine, paracrine, and endocrine) and at each stage (ligand, receptor, transducer, effector, terminator) in the cell signaling process. Thus, a cell signaling pathway cannot be fully described without understanding how intrinsically disordered protein regions contribute to its function. The ubiquitous presence of intrinsic disorder in different stages of diverse cell signaling pathways suggest that more mechanisms by which disorder modulates intra- and inter-cell signals remain to be discovered. Graphical abstract:

Overview

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TL;DR

This review argues that intrinsically disordered proteins/regions are pervasive and functionally critical across cell signaling, from ligands and receptors to transducers, effectors, and terminators, in all kingdoms and signaling categories. It synthesizes examples and mechanisms—binding-induced folding, fuzzy complexes, PTM/alternative-splicing integration, scaffolding, and allostery—showing that signaling pathways cannot be fully described without considering intrinsic disorder.

Key findings

  • IDPs/IDRs are present in signaling proteins across animals, plants, bacteria, and fungi, and in every signaling category (autocrine, juxtacrine, intracrine, paracrine, endocrine) and at each stage (ligand, receptor, transducer, effector, terminator).
  • Disorder enables key signaling properties: binding-induced folding decouples affinity from specificity; fuzzy complexes preserve disorder in bound states; low energetic barriers support reversible sensing; IDRs facilitate allosteric regulation, PTM/alternative-splicing integration (the IDP-AS-PTM toolkit), and scaffold-mediated amplification.
  • In the Wnt pathway analysis of 117 proteins, no proteins were very structured and only two were mostly disordered; the remaining 115 were moderately or highly disordered. Other examples include a 111-amino-acid intrinsically disordered region in the Notch intracellular domain, a 148-amino-acid disordered connexin C-terminal domain, an 852-residue mostly disordered axin scaffold, a 2,843-residue massively disordered APC protein, and Gab2 (74 kDa) with a ~550-amino-acid disordered region in which only two ~20-amino-acid regions bind Grb2.
  • Proteome predictions indicate ~15–30% of encoded residues are disordered in archaeal/eubacterial proteomes and ~30–50% in eukaryotic proteomes; top three disorder predictors achieved balanced accuracies of 76–80% on 646 proteins; >90% of transcription factors contain IDRs or are entirely disordered; and 15% of all disordered proteins bind lipid.

Methods

  • Narrative review, not primary research. The stated aims are to summarize best practices for identifying intrinsically disordered regions, highlight diverse cell signaling pathways that employ disordered proteins, and identify known molecular mechanisms implemented by these proteins and regions.
  • Evidence base includes literature examples across kingdoms and pathway categories, computational disorder predictions and tools (e.g., PONDR, IUPred, MoRF/ANCHOR predictors), and cited experimental studies; it includes a summary table of ID-enabled mechanisms.
  • No systematic search strategy, inclusion criteria, or meta-analysis is described in the text.

Limitations

  • The paper does not state a formal limitations section.
  • Evident from the text: it is a narrative review with illustrative examples rather than a systematic or exhaustive survey; some analyses are described as “work in progress” (e.g., protein interaction domains binding DBRs in IDRs), and some epigenetic implications are stated as unknown to the authors.
  • Evident: many mechanistic claims rely on computational predictions and cited secondary literature, and the review notes that more mechanisms by which disorder modulates signaling remain to be discovered.

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Registry

Crossref · 10.1186/s12964-022-00821-7

Cites

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Figures

Fig. 1 When binding a partner protein, intrinsically disordered regions can adopt multiple unstructured or structurally ambiguous topologies to form fuzzy complexes (A), fold to create stable secondary/tertiary structure (B) or adopt an unstructured yet static conformation (C)
Fig. 1 When binding a partner protein, intrinsically disordered regions can adopt multiple unstructured or structurally ambiguous topologies to form fuzzy complexes (A), fold to create stable secondary/tertiary structure (B) or adopt an unstructured yet static conformation (C)CC BY · PubMed Central
Fig. 2 Intrinsic disorder predisposition of human glucocorticoid receptor (UniProt ID: P04150) evaluated by PONDR® VSL2 [179], PONDR® VL3 [180], PONDR® VLXT [6], PONDR® FIT [181], IUPred2A_long and IUPred2A_short [181, 182]. Mean disorder score is shown as well. Positions of the N-terminal domain (residues 1–420), DNA binding domain (residues 420–485), a hinge region (residues 486–527), and ligand binding domain (residues 528–777) are shown as red, cyan, blue and lime shaded areas. Clearly, the N-terminal domain and a hinge region are mostly disordered
Fig. 2 Intrinsic disorder predisposition of human glucocorticoid receptor (UniProt ID: P04150) evaluated by PONDR® VSL2 [179], PONDR® VL3 [180], PONDR® VLXT [6], PONDR® FIT [181], IUPred2A_long and IUPred2A_short [181, 182]. Mean disorder score is shown as well. Positions of the N-terminal domain (residues 1–420), DNA binding domain (residues 420–485), a hinge region (residues 486–527), and ligand binding domain (residues 528–777) are shown as red, cyan, blue and lime shaded areas. Clearly, the N-terminal domain and a hinge region are mostly disorderedCC BY · PubMed Central
Fig. 3 The function of the glucocorticoid receptor is regulated in part by its intrinsically disordered C-terminal tail. A The GR tail interacts with chaperones in the cytoplasm in the unliganded state. B Alternative splicing and post-translational modifications impact these interactions
Fig. 3 The function of the glucocorticoid receptor is regulated in part by its intrinsically disordered C-terminal tail. A The GR tail interacts with chaperones in the cytoplasm in the unliganded state. B Alternative splicing and post-translational modifications impact these interactionsCC BY · PubMed Central
Fig. 4 Disorder can occur at any step of the Wnt cell signaling pathway. A A schematic of signaling components in the core canonical Wnt signaling pathway, showing the inactive state on the left and the active state on the right. The cell membrane is indicated by an arc and the nucleus by a light blue oval. Wnt signaling is able to control many processes by employing different variants of many proteins involved in Wnt signaling, which exist due to gene duplication, alternative splicing, and PTMs [258]. Consequently, a protein was marked as disordered (using a starburst) if the sequence off any variant, not post-translationally modified, was identified as intrinsically disordered in the literature. Wnt [258], Fz [268], LRP [268], Dsh/Dvl [264, 265], APC, CK1, Axin, GSK3 [269], β-catenin [269, 270], TCF/LEF [271], Groucho [272] all can include intrinsic disorder. B Analysis of 117 proteins involved in Wnt signaling based on disorder score and percent of disordered residues. Large values of each parameter indicate increasing disorder. Color blocks indicate regions in which are mostly ordered (blue and light blue), moderately disordered (pink), or mostly disordered (red). If the two parameters agree, the corresponding part of background is dark (blue or pink), whereas light blue and light pink reflect areas in which only one of these criteria applies. It is noteworthy that no Wnt pathway proteins are very structured (dark blue) and only two proteins can be considered mostly disordered. The remaining 115 proteins are either moderately disordered or highly disordered
Fig. 4 Disorder can occur at any step of the Wnt cell signaling pathway. A A schematic of signaling components in the core canonical Wnt signaling pathway, showing the inactive state on the left and the active state on the right. The cell membrane is indicated by an arc and the nucleus by a light blue oval. Wnt signaling is able to control many processes by employing different variants of many proteins involved in Wnt signaling, which exist due to gene duplication, alternative splicing, and PTMs [258]. Consequently, a protein was marked as disordered (using a starburst) if the sequence off any variant, not post-translationally modified, was identified as intrinsically disordered in the literature. Wnt [258], Fz [268], LRP [268], Dsh/Dvl [264, 265], APC, CK1, Axin, GSK3 [269], β-catenin [269, 270], TCF/LEF [271], Groucho [272] all can include intrinsic disorder. B Analysis of 117 proteins involved in Wnt signaling based on disorder score and percent of disordered residues. Large values of each parameter indicate increasing disorder. Color blocks indicate regions in which are mostly ordered (blue and light blue), moderately disordered (pink), or mostly disordered (red). If the two parameters agree, the corresponding part of background is dark (blue or pink), whereas light blue and light pink reflect areas in which only one of these criteria applies. It is noteworthy that no Wnt pathway proteins are very structured (dark blue) and only two proteins can be considered mostly disordered. The remaining 115 proteins are either moderately disordered or highly disorderedCC BY · PubMed Central
Fig. 5 Alternative splicing and PTMs, localized in intrinsically disordered regions, direct differential CXCR4 signaling. Predicted disorder identified by PONDR-FIT is depicted on a heat map (lower left), with red and blue indicating predicted disorder and order, respectively. A crystal structure of the structured regions (28–303 residues, PDB ID: 3OE9) is shown as a blue ribbon. Alternative splicing regulates receptor function by generating three tissue-specific isoforms by replacing the first five residues at the disordered N-terminus with other sequences of varying length. Multiple PTMs regulate different aspects of CXCR4 function: sulfation of Y7, Y12, and Y21 modulates receptor-ligand binding and dimerization [300], and glycosylation of N11 plays a role in masking the coreceptor functional activity [301]. Likewise, phosphorylation of Y157 is required for activation of the Gi-independent JAK2/STAT3 pathway [302]. Consequently, combinations of C-terminal PTMs are associated with three different biological processes: phosphorylation of S339 in G protein-coupled receptor kinase 6 (GRK6) and possibly GRK2 phosphorylation (two residues from S346-S348 and S351-S352) lead to receptor-arrestin3 binding, G protein uncoupling, and subsequent receptor desensitization. In contrast, phosphorylation of GRK3 (at the same regions as GRK2, but probably different residues), and GRK6 (S330 and S339) result in arrestin2 recruitment and subsequent ERK1/2 activation [303]. In addition, protein kinase C (PKC) and GRK6 phosphorylation (S324 or S325, S330 respectively) initiate degradation modulated by ubiquitination of K327, K331, and K333 [303, 304]. Adapted from Zhou et al. [39]
Fig. 5 Alternative splicing and PTMs, localized in intrinsically disordered regions, direct differential CXCR4 signaling. Predicted disorder identified by PONDR-FIT is depicted on a heat map (lower left), with red and blue indicating predicted disorder and order, respectively. A crystal structure of the structured regions (28–303 residues, PDB ID: 3OE9) is shown as a blue ribbon. Alternative splicing regulates receptor function by generating three tissue-specific isoforms by replacing the first five residues at the disordered N-terminus with other sequences of varying length. Multiple PTMs regulate different aspects of CXCR4 function: sulfation of Y7, Y12, and Y21 modulates receptor-ligand binding and dimerization [300], and glycosylation of N11 plays a role in masking the coreceptor functional activity [301]. Likewise, phosphorylation of Y157 is required for activation of the Gi-independent JAK2/STAT3 pathway [302]. Consequently, combinations of C-terminal PTMs are associated with three different biological processes: phosphorylation of S339 in G protein-coupled receptor kinase 6 (GRK6) and possibly GRK2 phosphorylation (two residues from S346-S348 and S351-S352) lead to receptor-arrestin3 binding, G protein uncoupling, and subsequent receptor desensitization. In contrast, phosphorylation of GRK3 (at the same regions as GRK2, but probably different residues), and GRK6 (S330 and S339) result in arrestin2 recruitment and subsequent ERK1/2 activation [303]. In addition, protein kinase C (PKC) and GRK6 phosphorylation (S324 or S325, S330 respectively) initiate degradation modulated by ubiquitination of K327, K331, and K333 [303, 304]. Adapted from Zhou et al. [39]CC BY · PubMed Central

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  • The paper relies on this without support.

    The examples reviewed are representative of cell signaling pathways generally.

    “IDPs/IDRs are uniquely suited to solving these problems, as demonstrated by several examples detailed in this review (Table 1).”
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Research works printed but not linked: 4

  1. Dunker AK, Obradovic Z, Romero P, Kissinger C, Villafranca JE. On the importance of being disordered. Protein Data Bank Q Newsl. 1997;81:3–5.
  2. Daughdrill GW, Pielak GJ, Uversky VN, Cortese MS, Dunker AK. Natively disordered proteins. In: Buchner J, Kiefhaber T, editors. Handbook of protein folding. Weinheim: Wiley, Verlag GmbH & Co KGaA; 2005. p. 271–353.
  3. Schulz GE. Nucleotide binding proteins. In: Balaban M, editor. Molecular mechanism of biological recognition. New York: Elsevier/North-Holland Biomedical Press; 1979. p. 79–94.
  4. Meek DW, Anderson CW. Post-translational modiication of p53” cooperative integrators of function. Cold Spring Harb Perspect Biol. 2009;1:a000950.

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Metadata

Authors
Bondos, S. E., Dunker, A. K., & Uversky, V. N.
Affiliations
  • Texas A&M Health Science Center (US)
  • Indiana University Indianapolis (US)
  • Russian Academy of Sciences (RU)
Corresponding author
Bondos, Sarah E.
Venue
Cell Communication and Signaling
Published
2022-02-17
Volume
20
Issue
1
Pages
20-20
ISSN
1478-811X (ISSN-L)
OpenAlex
W4212940766
Cited by
222
References
330
Citations by year
2026: 50 · 2025: 62 · 2024: 45 · 2023: 46 · 2022: 18
FWCI
4.45
Type
journal article
Language
en
Open access
Locations
  • Cell Communication and Signaling (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • IUScholarWorks (Indiana University) (submittedVersion, cc-by)
  • Digital Commons - University of South Florida (University of South Florida) (submittedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
  • National Institutes of Health (GM099827-06)
  • National Institute on Aging (RF1 AG055088)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Protein Structure and Dynamics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Receptor Mechanisms and Signaling
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Protein Kinase Regulation and GTPase Signaling
Keywords
intrinsically disordered proteins, intrinsically disordered regions, signal transduction, protein-protein interaction, ligand binding, receptor signaling, transducers, signal amplification, signal termination, autocrine signaling, paracrine signaling, hormone signaling, juxtacrine signaling, intracrine signaling, intercellular communication
MeSH terms
Animals, Cell Communication, Signal Transduction, Intrinsically Disordered Proteins

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