Structure-function relationships in protein homorepeats.

Homorepeats (or polyX), protein segments containing repetitions of the same amino acid, are abundant in proteomes from all kingdoms of life and are involved in crucial biological functions as well as several neurodegenerative and developmental diseases. Mainly inserted in disordered segments of proteins, the structure/function relationships of homorepeats remain largely unexplored. In this review, we summarize present knowledge for the most abundant homorepeats, highlighting the role of the inherent structure and the conformational influence exerted by their flanking regions. Recent experimental and computational methods enable residue-specific investigations of these regions and promise novel structural and dynamic information for this elusive group of proteins. This information should increase our knowledge about the structural bases of phenomena such as liquid-liquid phase separation and trinucleotide repeat disorders.

Site-Specific Introduction of Alanines for the Nuclear Magnetic Resonance Investigation of Low-Complexity Regions and Large Biomolecular Assemblies.

Nuclear magnetic resonance (NMR) studies of large biomolecular machines and highly repetitive proteins remain challenging due to the difficulty of assigning frequencies to individual nuclei. Here, we present an efficient strategy to address this challenge by engineering a Pyrococcus horikoshii tRNA/alanyl-tRNA synthetase pair that enables the incorporation of up to three isotopically labeled alanine residues in a site-specific manner using in vitro protein expression. The general applicability of this approach for NMR assignment has been demonstrated by introducing isotopically labeled alanines into four distinct proteins: huntingtin exon-1, HMA8 ATPase, the 300 kDa molecular chaperone ClpP, and the alanine-rich Phox2B transcription factor. For large protein assemblies, our labeling approach enabled unambiguous assignments while avoiding potential artifacts induced by site-specific mutations. When applied to Phox2B, which contains two poly-alanine tracts of nine and twenty alanines, we observed that the helical stability is strongly dependent on the homorepeat length. The capacity to selectively introduce alanines with distinct labeling patterns is a powerful tool to probe structure and dynamics of challenging biomolecular systems.

Multi-site-specific isotopic labeling accelerates high-resolution structural investigations of pathogenic huntingtin exon-1.

Huntington's disease neurodegeneration occurs when the number of consecutive glutamines in the huntingtin exon-1 (HTTExon1) exceeds a pathological threshold of 35. The sequence homogeneity of HTTExon1 reduces the signal dispersion in NMR spectra, hampering its structural characterization. By simultaneously introducing three isotopically labeled glutamines in a site-specific manner in multiple concatenated samples, 18 glutamines of a pathogenic HTTExon1 with 36 glutamines were unambiguously assigned. Chemical shift analyses indicate the α-helical persistence in the homorepeat and the absence of an emerging toxic conformation around the pathological threshold. Using the same type of samples, the recognition mechanism of Hsc70 molecular chaperone has been investigated, indicating that it binds to the N17 region of HTTExon1, inducing the partial unfolding of the poly-Q. The proposed strategy facilitates high-resolution structural and functional studies in low-complexity regions.

The structure of pathogenic huntingtin exon 1 defines the bases of its aggregation propensity.

Huntington's disease is a neurodegenerative disorder caused by a CAG expansion in the first exon of the HTT gene, resulting in an extended polyglutamine (poly-Q) tract in huntingtin (httex1). The structural changes occurring to the poly-Q when increasing its length remain poorly understood due to its intrinsic flexibility and the strong compositional bias. The systematic application of site-specific isotopic labeling has enabled residue-specific NMR investigations of the poly-Q tract of pathogenic httex1 variants with 46 and 66 consecutive glutamines. Integrative data analysis reveals that the poly-Q tract adopts long α-helical conformations propagated and stabilized by glutamine side chain to backbone hydrogen bonds. We show that α-helical stability is a stronger signature in defining aggregation kinetics and the structure of the resulting fibrils than the number of glutamines. Our observations provide a structural perspective of the pathogenicity of expanded httex1 and pave the way to a deeper understanding of poly-Q-related diseases.

Nucleus-translocated mitochondrial cytochrome c liberates nucleophosmin-sequestered ARF tumor suppressor by changing nucleolar liquid-liquid phase separation.

The regular functioning of the nucleolus and nucleus-mitochondria crosstalk are considered unrelated processes, yet cytochrome c (Cc) migrates to the nucleus and even the nucleolus under stress conditions. Nucleolar liquid-liquid phase separation usually serves the cell as a fast, smart mechanism to control the spatial localization and trafficking of nuclear proteins. Actually, the alternative reading frame (ARF), a tumor suppressor protein sequestered by nucleophosmin (NPM) in the nucleoli, is shifted out from NPM upon DNA damage. DNA damage also triggers early translocation of respiratory Cc to nucleus before cytoplasmic caspase activation. Here, we show that Cc can bind to nucleolar NPM by triggering an extended-to-compact conformational change, driving ARF release. Such a NPM-Cc nucleolar interaction can be extended to a general mechanism for DNA damage in which the lysine-rich regions of Cc-rather than the canonical, arginine-rich stretches of membrane-less organelle components-controls the trafficking and availability of nucleolar proteins.

The sequence context in poly-alanine regions: structure, function and conservation.

MOTIVATION: Poly-alanine (polyA) regions are protein stretches mostly composed of alanines. Despite their abundance in eukaryotic proteomes and their association to nine inherited human diseases, the structural and functional roles exerted by polyA stretches remain poorly understood. In this work we study how the amino acid context in which polyA regions are settled in proteins influences their structure and function. RESULTS: We identified glycine and proline as the most abundant amino acids within polyA and in the flanking regions of polyA tracts, in human proteins as well as in 17 additional eukaryotic species. Our analyses indicate that the non-structuring nature of these two amino acids influences the α-helical conformations predicted for polyA, suggesting a relevant role in reducing the inherent aggregation propensity of long polyA. Then, we show how polyA position in protein N-termini relates with their function as transit peptides. PolyA placed just after the initial methionine is often predicted as part of mitochondrial transit peptides, whereas when placed in downstream positions, polyA are part of signal peptides. A few examples from known structures suggest that short polyA can emerge by alanine substitutions in α-helices; but evolution by insertion is observed for longer polyA. Our results showcase the importance of studying the sequence context of homorepeats as a mechanism to shape their structure-function relationships. AVAILABILITY AND IMPLEMENTATION: The datasets used and/or analyzed during the current study are available from the corresponding author onreasonable request. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

PP2A is activated by cytochrome c upon formation of a diffuse encounter complex with SET/TAF-Iß.

Intrinsic protein flexibility is of overwhelming relevance for intermolecular recognition and adaptability of highly dynamic ensemble of complexes, and the phenomenon is essential for the understanding of numerous biological processes. These conformational ensembles-encounter complexes-lack a unique organization, which prevents the determination of well-defined high resolution structures. This is the case for complexes involving the oncoprotein SET/template-activating factor-Iß (SET/TAF-Iß), a histone chaperone whose functions and interactions are significantly affected by its intrinsic structural plasticity. Besides its role in chromatin remodeling, SET/TAF-Iß is an inhibitor of protein phosphatase 2A (PP2A), which is a key phosphatase counteracting transcription and signaling events controlling the activity of DNA damage response (DDR) mediators. During DDR, SET/TAF-Iß is sequestered by cytochrome c (Cc) upon migration of the hemeprotein from mitochondria to the cell nucleus. Here, we report that the nuclear SET/TAF-Iß:Cc polyconformational ensemble is able to activate PP2A. In particular, the N-end folded, globular region of SET/TAF-Iß (a.k.a. SET/TAF-Iß ΔC)-which exhibits an unexpected, intrinsically highly dynamic behavior-is sufficient to be recognized by Cc in a diffuse encounter manner. Cc-mediated blocking of PP2A inhibition is deciphered using an integrated structural and computational approach, combining small-angle X-ray scattering, electron paramagnetic resonance, nuclear magnetic resonance, calorimetry and molecular dynamics simulations.

Proposed mechanism for regulation of H2 O2 -induced programmed cell death in plants by binding of cytochrome c to 14-3-3 proteins.

Programmed cell death (PCD) is crucial for development and homeostasis of all multicellular organisms. In human cells, the double role of extra-mitochondrial cytochrome c in triggering apoptosis and inhibiting survival pathways is well reported. In plants, however, the specific role of cytochrome c upon release from the mitochondria remains in part veiled yet death stimuli do trigger cytochrome c translocation as well. Here, we identify an Arabidopsis thaliana 14-3-3ι isoform as a cytosolic cytochrome c target and inhibitor of caspase-like activity. This finding establishes the 14-3-3ι protein as a relevant factor at the onset of plant H2 O2 -induced PCD. The in vivo and in vitro studies herein reported reveal that the interaction between cytochrome c and 14-3-3ι exhibits noticeable similarities with the complex formed by their human orthologues. Further analysis of the heterologous complexes between human and plant cytochrome c with plant 14-3-3ι and human 14-3-3ε isoforms corroborated common features. These results suggest that cytochrome c blocks p14-3-3ι so as to inhibit caspase-like proteases, which in turn promote cell death upon H2 O2 treatment. Besides establishing common biochemical features between human and plant PCD, this work sheds light onto the signaling networks of plant cell death.

Robust Cell-Free Expression of Sub-Pathological and Pathological Huntingtin Exon-1 for NMR Studies. General Approaches for the Isotopic Labeling of Low-Complexity Proteins.

The high-resolution structural study of huntingtin exon-1 (HttEx1) has long been hampered by its intrinsic properties. In addition to being prone to aggregate, HttEx1 contains low-complexity regions (LCRs) and is intrinsically disordered, ruling out several standard structural biology approaches. Here, we use a cell-free (CF) protein expression system to robustly and rapidly synthesize (sub-) pathological HttEx1. The open nature of the CF reaction allows the application of different isotopic labeling schemes, making HttEx1 amenable for nuclear magnetic resonance studies. While uniform and selective labeling facilitate the sequential assignment of HttEx1, combining CF expression with nonsense suppression allows the site-specific incorporation of a single labeled residue, making possible the detailed investigation of the LCRs. To optimize CF suppression yields, we analyze the expression and suppression kinetics, revealing that high concentrations of loaded suppressor tRNA have a negative impact on the final reaction yield. The optimized CF protein expression and suppression system is very versatile and well suited to produce challenging proteins with LCRs in order to enable the characterization of their structure and dynamics.

Flanking Regions Determine the Structure of the Poly-Glutamine in Huntingtin through Mechanisms Common among Glutamine-Rich Human Proteins.

The causative agent of Huntington's disease, the poly-Q homo-repeat in the N-terminal region of huntingtin (httex1), is flanked by a 17-residue-long fragment (N17) and a proline-rich region (PRR), which promote and inhibit the aggregation propensity of the protein, respectively, by poorly understood mechanisms. Based on experimental data obtained from site-specifically labeled NMR samples, we derived an ensemble model of httex1 that identified both flanking regions as opposing poly-Q secondary structure promoters. While N17 triggers helicity through a promiscuous hydrogen bond network involving the side chains of the first glutamines in the poly-Q tract, the PRR promotes extended conformations in neighboring glutamines. Furthermore, a bioinformatics analysis of the human proteome showed that these structural traits are present in many human glutamine-rich proteins and that they are more prevalent in proteins with longer poly-Q tracts. Taken together, these observations provide the structural bases to understand previous biophysical and functional data on httex1.

Evidence of the Reduced Abundance of Proline cis Conformation in Protein Poly Proline Tracts.

Proline is found in a cis conformation in proteins more often than other proteinogenic amino acids, where it influences structure and modulates function, being the focus of several high-resolution structural studies. However, until now, technical and methodological limitations have hampered the site-specific investigation of the conformational preferences of prolines present in poly proline (poly-P) homorepeats in their protein context. Here, we apply site-specific isotopic labeling to obtain high-resolution NMR data on the cis/trans equilibrium of prolines within the poly-P repeats of huntingtin exon 1, the causative agent of Huntington's disease. Screening prolines in different positions in long (poly-P11) and short (poly-P3) poly-P tracts, we found that, while the first proline of poly-P tracts adopts similar levels of cis conformation as isolated prolines, a length-dependent reduced abundance of cis conformers is observed for terminal prolines. Interestingly, the cis isomer could not be detected in inner prolines, in line with percentages derived from a large database of proline-centered tripeptides extracted from crystallographic structures. These results suggest a strong cooperative effect within poly-Ps that enhances their stiffness by diminishing the stability of the cis conformation. This rigidity is key to rationalizing the protection toward aggregation that the poly-P tract confers to huntingtin. Furthermore, the study provides new avenues to probe the structural properties of poly-P tracts in protein design as scaffolds or nanoscale rulers.

Site-Specific Isotopic Labeling (SSIL): Access to High-Resolution Structural and Dynamic Information in Low-Complexity Proteins.

Remarkable technical progress in the area of structural biology has paved the way to study previously inaccessible targets. For example, large protein complexes can now be easily investigated by cryo-electron microscopy, and modern high-field NMR magnets have challenged the limits of high-resolution characterization of proteins in solution. However, the structural and dynamic characteristics of certain proteins with important functions still cannot be probed by conventional methods. These proteins in question contain low-complexity regions (LCRs), compositionally biased sequences where only a limited number of amino acids is repeated multiple times, which hamper their characterization. This Concept article describes a site-specific isotopic labeling (SSIL) strategy, which combines nonsense suppression and cell-free protein synthesis to overcome these limitations. An overview on how poly-glutamine tracts were made amenable to high-resolution structural studies is used to illustrate the usefulness of SSIL. Furthermore, we discuss the potential of this methodology to give further insights into the roles of LCRs in human pathologies and liquid-liquid phase separation, as well as the challenges that must be addressed in the future for the popularization of SSIL.

Oxidative stress is tightly regulated by cytochrome c phosphorylation and respirasome factors in mitochondria.

Respiratory cytochrome c has been found to be phosphorylated at tyrosine 97 in the postischemic brain upon neuroprotective insulin treatment, but how such posttranslational modification affects mitochondrial metabolism is unclear. Here, we report the structural features and functional behavior of a phosphomimetic cytochrome c mutant, which was generated by site-specific incorporation at position 97 of p-carboxymethyl-l-phenylalanine using the evolved tRNA synthetase method. We found that the point mutation does not alter the overall folding and heme environment of cytochrome c, but significantly affects the entire oxidative phosphorylation process. In fact, the electron donation rate of the mutant heme protein to cytochrome c oxidase, or complex IV, within respiratory supercomplexes was higher than that of the wild-type species, in agreement with the observed decrease in reactive oxygen species production. Direct contact of cytochrome c with the respiratory supercomplex factor HIGD1A (hypoxia-inducible domain family member 1A) is reported here, with the mutant heme protein exhibiting a lower affinity than the wild-type species. Interestingly, phosphomimetic cytochrome c also exhibited a lower caspase-3 activation activity. Altogether, these findings yield a better understanding of the molecular basis for mitochondrial metabolism in acute diseases, such as brain ischemia, and thus could allow the use of phosphomimetic cytochrome c as a neuroprotector with therapeutic applications.

Cytochrome c speeds up caspase cascade activation by blocking 14-3-3ε-dependent Apaf-1 inhibition.

Apoptosis is a highly regulated form of programmed cell death, essential to the development and homeostasis of multicellular organisms. Cytochrome c is a central figure in the activation of the apoptotic intrinsic pathway, thereby activating the caspase cascade through its interaction with Apaf-1. Our recent studies have revealed 14-3-3ε (a direct inhibitor of Apaf-1) as a cytosolic cytochrome c target. Here we explore the cytochrome c / 14-3-3ε interaction and show the ability of cytochrome c to block 14-3-3ε-mediated Apaf-1 inhibition, thereby unveiling a novel function for cytochrome c as an indirect activator of caspase-9/3. We have used calorimetry, NMR spectroscopy, site mutagenesis and computational calculations to provide an insight into the structural features of the cytochrome c / 14-3-3ε complex. Overall, these findings suggest an additional cytochrome c-mediated mechanism to modulate apoptosome formation, shedding light onto the rigorous apoptotic regulation network.