Proteins are social beings. Especially disordered proteins like company, they hardly
act without interacting with another macromolecule. In most known cases, these other
macromolecules are other proteins, sometimes nucleic acids and very seldom something
else. Disordered proteins are rather newcomers in protein science. The first papers
on these proteins came out in the fourth quarter of the last century. What is more,
they were hardly recognized before the great paper of Wright and Dyson published in
J. Mol. Biol. in 1999 [1]. By now, it is well known that a large portion of all existing
proteins are intrinsically disordered under physiological conditions. They perform
vital roles in many living cells. For more than a decade, it was generally thought
that disordered proteins or disorder parts of partially disordered proteins have different
amino acid composition than folded proteins have and various prediction methods were
developed based on this principle. Dosztanyi et al. [2,3] provided a physical background
of a disorder prediction methods (IUPred) by estimating the lowest value of the sum
of the pairwise interaction energies between residues from the amino acid sequences
without considering structural information. This calculated energy per residue value
for globular proteins was well separated from the ones calculated for disordered (unstructured)
proteins know at that time. This principle of pair energy estimation applied in IUPred
also worked well, when the method ANCHOR [4,5] was developed to predict binding site
within disordered parts of protein by which the disordered protein bounds to a folded
one and its structure is formed upon binding. Those segments of the disordered proteins
are identified as binding site where in amino acids, considering together with the
average composition of folded protein, exhibit low enough pairwise interaction energy
to be stable. Recently however shreds of evidence were accumulated about the existence
of a different type of disordered proteins [6]. It turned out that some disordered
proteins can undergo coupled folding and binding without the involvement of an already
folded protein, but by intra-acting with disordered proteins. This second protein
can be the same as the first one (formation of homodimers) or can be different (formation
of heterodimers. They can also form higher order oligomers. These proteins which can
stabilize their structure via “mutual synergistic folding” have residue compositions
similar to that of the folded globular water-soluble proteins. Their residue compositions
are different from the composition of the traditional disordered proteins, which can
only be stabilized on the surface of an already stabilized macromolecule, in most
cases on the surface of a folded protein. These traditional disordered proteins can
be named as “coupled folding and binding” protein. Recently the “mutual synergistic
folding” proteins were collected in a database MFIB [7], the “coupled folding and
binding” proteins were collected in a database DIBS [8] and the structural and functional
properties of these two types of protein were compared [9]. Beside the large variation
of protein-protein interactions, in the past decade, more and more examples are found,
where disordered proteins interact with non-protein macromolecules in various forms
[10]. There is also a very new phenomenon when proteins, including disordered ones,
are involved in phase separation, which can be a weak but functionally important macromolecular
interaction [11].
Paper of two merged special issues on the same topic: “Functionally relevant macromolecular
interactions of disordered proteins” are summarized and listed in the order of their
online publication date. Research, review and concept papers listed separately, starting
with the oldest one in this Editorial.
1. Research
In the first paper of this series, a study on the effect of acetylation on the phase
separation tendency of Tau protein was reported by Ferreon et al. It is well known
that intrinsically disordered protein Tau is involved in Alzheimer’s disease. Recently
it was shown that Tau is capable of undergoing liquid-liquid phase separation, which
involves weak protein-protein interactions and it is considered as an initiation of
Tau aggregation observed in Alzheimer’s disease. In this work, it was shown that acetylation
disfavors phase separation and aggregation of Tau, therefore, acetylation prevents
the toxic effects of liquid-liquid phase separation dependent aggregation [12].
Srivastava et al. tried to decipher RNA-recognition patterns of IDPs in the next paper.
They analyzed the protein-RNA complexes which undergo disordered to ordered transition
(DOT) during binding. The DOT region is small and positively charged, like the binding
sites in globular proteins. However, for DOTs of IDP have significantly higher exposure
to the water, than their counterpart in structured protein. These findings can help
to develop tools for identifying DOT regions in RNA binding proteins [13].
Contreras et al. studied a Protein (LrtA protein of Synechocystis sp. PCC 6803) which
is oligomeric and folded in solution, but the single-chain is only folded and stable
in their N terminal half of the polypeptide (residues 1–100) while the other half
(101–197) is very unstable and rather disordered with chameleonic sequence properties.
While disordered protein which undergoes mutual synergistic folding upon binding to
each other, this is a rather rear case, when it happens only with half of the protein.
The other half remains folded before and after self-association [14].
The origin of the thermal stability of eukaryotic proteins was studied and compared
with that of thermophilic and mesophilic proteins of prokaryotes by Alvarez-Ponce
et al. The eukaryotic model system was Arabidopsis thaliana at 22 and 37 °C, and they
compare both the amino acid compositions and levels of intrinsic disorder of heat-induced
and heat-repressed proteins. Heat-induced proteins are enriched in intrinsically disordered
regions and depleted in hydrophobic amino acids in contrast to thermophile prokaryotic
proteins [15].
A decision-tree based meta server to predict disordered parts of proteins and their
residues involved in binding motifs has been developed by Zhao and Xue. The meta server
is based on four predictors: DisEMBL, IUPred, VSL2, and ESpritz. The meta server provides
higher accuracy than each of these independent predictors [16].
Arvidsson and Wright applied a protein disorder approach characterizing differentially
expressed genes analyzing cell adhesion regulated gene expression in lymphoma cells.
They checked if predicted protein disorder was differentially associated with proteins
encoded by differentially regulated genes in lymphoma cells. Intrinsic disorder protein
properties were extracted from the Database of Disordered Protein Prediction (D2P2).
They concluded that down-regulated genes in stromal cell-adherent lymphoma cells encode
proteins that are characterized by elevated levels of disorder [17].
The co-evolution of IDPs and folded partner proteins was studied by checking their
evolutionary couplings. Pancsa et al. pointed that due to the lack of strict structural
constraints, IDPs undergo faster evolutionary changes than folded proteins, which
makes the reliable identification and alignment of IDP homologs difficult. They demonstrated
that partner binding imposes constraints on IDP sequences that manifest in detectable
inter-protein evolutionary couplings. It brings hope that IDP–partner interactions
could soon be successfully dissected through residue co-variation analysis [18].
A principal part of the physical bases of disordered proteins involved in mutual synergetic
folding in homodimers has been uncovered by Magyar et al. The authors concluded that
homodimer proteins have a larger solvent-accessible main-chain surface area on the
contact surface of the subunits, when compared to globular homodimer proteins. The
main driving force of the dimerization is the mutual shielding of the water-accessible
backbones and the formation of extra intermolecular interactions [19].
Szabó et al. reported their finding that disordered parts of Mixed Lineage Leukemia
4 (MLL4) protein are capable of RNA binding. They explored the RNA binding capability
of two; uncharacterized regions of MLL4; with the aim of shedding light to the existence
of possible regulatory lncRNA interactions of the protein They demonstrated that both
regions; one that contains a predicted RNA binding sequence and one that does not,
are capable of binding to different RNA constructs in vitro [20].
A method to characterize the hydration of proteins based on evaluating two-component
wide-line 1H NMR signals is presented in the next paper. Tompa et al. also provided
a description of key elements of the procedure conceived for the thermodynamic interpretation
of such results. The results enable a quantitative description of the ratio of ordered
and disordered parts of proteins, and the energy relations of protein–water bonds
in aqueous solutions of the proteins [21].
Homma et al, studied the evolution rate structural domains (SDs) and intrinsically
disordered regions (IDRs) of immune-related mammalian proteins. IDRs are generally
subject to fewer constraints and evolve more rapidly than SDs. However, it turned
out that for immune-related proteins in mammals, the evolution rates in SDs come close
to those in IDRs [22].
Moosa, M.M. et al. applied direct single-molecule observation to study sequential
DNA bending transitions by the SoxSox2 is a transcription factor which assumed to
achieve its regulatory diversity via heterodimerization with partner transcription
factors. However, single-molecule fluorescence spectroscopy suggests that Sox2 alone
can modulate structural landscape of the DNA in a dosage-dependent manner [23].
In a paper which was a follow-up of the Contreras, L.M. et al. paper [14], Neira et
al. reported a study on the structure of the C-terminal half (residues 102–191) of
the LrtA protein of Synechocystis sp. PCC 6803 in separated form with various physical-chemical
techniques. At physiological conditions isolated C-LrtA intervened in a self-association
equilibrium, involving several oligomerization reactions. They concluded that C-LrtA
was an oligomeric disordered protein [24].
Mishra et al. extended their one-bead-per-amino-acid model for intrinsically disordered
proteins to account for phosphorylation in studying the effect of phosphorylation
on nuclear pore complex selectivity. The simulations show that upon phosphorylation
the transport rate of inert molecules increases, while that of nuclear transport receptors
decreases. The models provide a molecular framework to explain how extensive phosphorylation
decreases the selectivity of the nuclear pore complexes [25].
Walter et al. studied the hydrodynamic properties of the intrinsically disordered
potyvirus genome-linked protein, VVPg), of the translation initiation factor, eIF4E,
and of their binary complex (VPg)-eIF4E. N-terminal His tag decreased the conformational
entropy of this intrinsically disordered region. A comparative study revealed the
His tag contribution to the hydrodynamic behavior of proteins [26].
The role of intrinsically disordered linkers in the confinement of binding domains
in enzyme actions was studied in the following paper. By statistical physical modeling
Szabo et al. show that this arrangement results in processive systems, in which the
linker ensures an optimized effective concentration around novel the binding site(s),
favoring rebinding over full release of the polymeric partner. By analyzing 12 enzymes
they suggest a unique type of entropic chain function of intrinsically disordered
proteins, that may impart functional advantages on diverse enzymes in a variety of
biological contexts [27].
Machulin et al. studied the contribution of repeats in ribosomal S1 proteins into
the tendency for intrinsic disorder and flexibility within and between structural
domains for all available UniProt S1 sequences. Using charge–hydrophobicity plot cumulative
distribution function (CH-CDF) analysis they classified 53% of S1 proteins as ordered
proteins, the remaining proteins were related to molten globule state. According to
the FoldUnfold and IsUnstruct programs, relatively short flexible or disordered regions
are predominant in the multi-domain proteins. Their results suggest that the ratio
of flexibility in the separate domains is related to their roles in the activity and
functionality of S1 [28].
The decrease of disorder level of p53-DBD upon interacting with the anticancer protein
Azurin by mean of Raman spectroscopy was monitored by Signorelli et al. This technique
was found to be suitable to elucidate the structural properties of intrinsically disordered
proteins and was applied to investigate the changes in both the structure and the
conformational heterogeneity of the DNA-binding domain (DBD) belonging to the intrinsically
disordered protein p53 upon its binding to Azurin, an electron-transfer anticancer
protein from Pseudomonas aeruginosa. The results show an increase of the secondary
structure content of DBD concomitantly with a decrease of its conformational heterogeneity
upon its binding to Azurin [29].
Structural and functional properties of a capsid protein of dengue and related flavivirus.
Dengue, West Nile and Zika have very similar viral particle with an outer lipid bilayer
containing two viral proteins in the nucleocapsid core were studied by Faustino et
al. Using dengue virus capsid protein as the main model, the protein size, thermal
stability, and function with its structure/dynamics features were correlated. Their
findings suggest that the capsid protein interaction with host lipid systems leads
to minor allosteric changes that may modulate the specific binding of the protein
to the viral RNA [30].
Chan-Yao-Chong, et al. investigated the early steps of actin recognition of Neural
Wiskott–Aldrich Syndrome Protein (N-WASP) domain V. Using docking calculations and
molecular dynamics simulations, their study shows that actin is first recognized by
the N-WASP domain V regions which have the highest propensity to form transient α
–helices. The WH2 motif consensus sequences “LKKV” subsequently binds to actin through
large conformational changes of the disordered domain V [31].
Mentes et al.’s paper is the follow-up of the Magyar’s paper [19] of this collection.
It reports the properties of heterodimer Mutual Synergistic Folding (MSF) proteins
instead of homodimeric ones. The main driving force of the dimerization is the mutual
shielding of the water-accessible backbones and the formation of extra intermolecular
interactions just like in homodimers. However here shielding of the β-sheet backbones
and the formation of a buried structural core along with the general strengthening
of inter-subunit interactions together could be important factors [32].
Conformational ensembles of alpha-Synuclein were studied using single-molecule force
spectroscopy and mass spectroscopy by Corti et al. This work applies single-molecule
force spectroscopy to probe conformational properties of α-synuclein in solution and
its conformational changes induced by ligand binding. This analysis provides support
to the structural interpretation of charge-state distributions obtained by native
mass spectrometry and helps defining the conformational components detected by single-molecule
force spectroscopy [33].
The topic of the Mészáros et al. paper is closely related to the ones of the Mentes
et al’s. paper [32] and the Magyar et al. paper [19]. The authors report the sequence
and structure properties of protein complexes formed by disordered proteins via Mutual
Synergistic Folding (MSF). A method is presented which differences in binding strength,
subcellular localization, and regulation are encoded in the sequence and structural
properties of proteins. It serves as a better representation of structures arising
through this specific interaction mode [34].
Three Rett syndromes (RTT) treatment-related genes MECP2, CDKL5 and FOXG1 in silico
by evolutionary classification and disordered region assessment were reported in this
paper. Fahmi, M. et al. provided insight into the structural characteristics, evolution
and interaction landscapes of those three proteins. They also reported the disordered
structure properties and evolution of those proteins which may provide valuable information
for the development of therapeutic strategies of RTT [35].
2. Review
Sánchez-López et al. report about the structural determinants of the N-terminus of
the prion protein and the effect of binding copper ions in their review. They discuss
the current knowledge of how mutations can impact the copper-binding properties of
prion protein both in health and disease progression [36].
In their review paper Ciemny et al. first introduced the technique of Monte Carlo
and other simulation for predicting possible structures of unstructured protein, protein-peptide
complexes and unfolded states of globular proteins. They presented several case studies
on various disordered proteins. They also proposed the use of the CABS coarse-grained
model with Monte Carlo sampling scheme. They also show that CABS can be combined with
the use of experimental data too [37].
The literature information on the alteration of disordered proteins in neurodegenerative
and other diseases reported in this review. Martinelli et al. discussed how the misfolded
proteins can be involved in Alzheimer’s, Parkinson’s and other diseases. The most
common form of misfolding IDPs is the formation of neurotoxic amyloid plaques. The
review discusses important special cases of beta-amyloid, alpha-synuclein, tau etc.
They also show drug candidates for later use in to treatment of diseases caused by
misfolded IDPs [38].
This is the first review from the Greb-Markiewicz lab in which Kolonko and Greb-Markiewicz
summarized our current knowledge on helix-loop-helix/Per-ARNT-SIM (bHLH–PAS) proteins,
considering their structures and intrinsic disorder nature based on NMR and X-ray
analysis. Currently, all determined structures comprise only selected domains (bHLH
and/or PAS), while parts of proteins, comprising their long C-termini, have not been
structurally characterized yet since these regions appear to be intrinsically disordered.
These intrinsically disordered parts contribute a lot to the flexibility and function
of these proteins [39].
The second review from the same lab is the paper of Tarczewska and Greb-Markiewicz
which is a follow-up publication of the review paper of Kolonko and Greb-Markiewicz
[39], the currently available information on “The Significance of the Intrinsically
Disordered Regions for the Function of the BHLH Transcription Factors” is reported.
Their aim was to emphasize the significance of existing disordered regions within
the helix–loop–helix (bHLH) transcription factors for their functionality [40].
Finally, in the last review paper of this collection, Owen and Shewmaker summarized
our current knowledge on “The Role of Post-Translational Modification in the Phase
Transition of Intrinsically Disordered Proteins”. They pointed that intrinsically
disordered regions are critical to the liquid–liquid phase separation that facilitates
specialized cellular functions and discuss how post-translational modifications of
intrinsically disordered protein segments can regulate the molecular condensation
of macromolecules into functional phase-separated complexes [41].
3. Concept
In his concept paper, Rikkerin, E.H.A. considers the “first line response” role of
disordered protein in the protection against pathogens and disease. He presents several
examples of how disorder and post-translational changes can play in the response of
organisms to the stress of a changing environment. He proposes that some disordered
proteins enable organisms to sense and react rapidly as the first line responds [42].