Persistence of Methionine Side Chain Mobility at Low Temperatures in a Nine-Residue Low Complexity Peptide, as Probed by 2 H Solid-State NMR.

Methionine side chains are flexible entities which play important roles in defining hydrophobic interfaces. We utilize deuterium static solid-state NMR to assess rotameric inter-conversions and other dynamic modes of the methionine in the context of a nine-residue random-coil peptide (RC9) with the low-complexity sequence GGKGMGFGL. The measurements in the temperature range of 313 to 161 K demonstrate that the rotameric interconversions in the hydrated solid powder state persist to temperatures below 200 K. Removal of solvation significantly reduces the rate of the rotameric motions. We employed 2 H NMR line shape analysis, longitudinal and rotation frame relaxation, and chemical exchange saturation transfer methods and found that the combination of multiple techniques creates a significantly more refined model in comparison with a single technique. Further, we compare the most essential features of the dynamics in RC9 to two different methionine-containing systems, characterized previously. Namely, the M35 of hydrated amyloid-ß1-40 in the three-fold symmetric polymorph as well as Fluorenylmethyloxycarbonyl (FMOC)-methionine amino acid with the bulky hydrophobic group. The comparison suggests that the driving force for the enhanced methionine side chain mobility in RC9 is the thermodynamic factor stemming from distributions of rotameric populations, rather than the increase in the rate constant.

Nine-residue low-complexity disordered peptide as a model system, an NMR/CD study.

Disordered proteins and protein segments can be crucial for biological function. In this work we present a detailed biophysical characterization of the low-complexity nine-residue peptide with the sequence GGKGMGFGL. Based on proton solution NMR chemical shifts, circular dichroism measurements, as well as the analysis of concentration dependence of NMR linewidth, proton longitudinal relaxation times, hydrogen-deuterium exchange measurements, and 15N rotating frame NMR relaxation measurements, we conclude that the peptide is fully disordered and monomeric in solution. The peptide will serve as a model system for future structural and dynamics studies of biologically relevant disordered peptides in solution and solid states.

Sortase-mediated segmental labeling: A method for segmental assignment of intrinsically disordered regions in proteins.

A significant number of proteins possess sizable intrinsically disordered regions (IDRs). Due to the dynamic nature of IDRs, NMR spectroscopy is often the tool of choice for characterizing these segments. However, the application of NMR to IDRs is often hindered by their instability, spectral overlap and resonance assignment difficulties. Notably, these challenges increase considerably with the size of the IDR. In response to these issues, here we report the use of sortase-mediated ligation (SML) for segmental isotopic labeling of IDR-containing samples. Specifically, we have developed a ligation strategy involving a key segment of the large IDR and adjacent folded headpiece domain comprising the C-terminus of A. thaliana villin 4 (AtVLN4). This procedure significantly reduces the complexity of NMR spectra and enables group identification of signals arising from the labeled IDR fragment, a process we refer to as segmental assignment. The validity of our segmental assignment approach is corroborated by backbone residue-specific assignment of the IDR using a minimal set of standard heteronuclear NMR methods. Using segmental assignment, we further demonstrate that the IDR region adjacent to the headpiece exhibits nonuniform spectral alterations in response to temperature. Subsequent residue-specific characterization revealed two segments within the IDR that responded to temperature in markedly different ways. Overall, this study represents an important step toward the selective labeling and probing of target segments within much larger IDR contexts. Additionally, the approach described offers significant savings in NMR recording time, a valuable advantage for the study of unstable IDRs, their binding interfaces, and functional mechanisms.

Oxidative damage to epigenetically methylated sites affects DNA stability, dynamics and enzymatic demethylation.

DNA damage can affect various regulatory elements of the genome, with the consequences for DNA structure, dynamics, and interaction with proteins remaining largely unexplored. We used solution NMR spectroscopy, restrained and free molecular dynamics to obtain the structures and investigate dominant motions for a set of DNA duplexes containing CpG sites permuted with combinations of 5-methylcytosine (mC), the primary epigenetic base, and 8-oxoguanine (oxoG), an abundant DNA lesion. Guanine oxidation significantly changed the motion in both hemimethylated and fully methylated DNA, increased base pair breathing, induced BI→BII transition in the backbone 3' to the oxoG and reduced the variability of shift and tilt helical parameters. UV melting experiments corroborated the NMR and molecular dynamics results, showing significant destabilization of all methylated contexts by oxoG. Notably, some dynamic and thermodynamic effects were not additive in the fully methylated oxidized CpG, indicating that the introduced modifications interact with each other. Finally, we show that the presence of oxoG biases the recognition of methylated CpG dinucleotides by ROS1, a plant enzyme involved in epigenetic DNA demethylation, in favor of the oxidized DNA strand. Thus, the conformational and dynamic effects of spurious DNA oxidation in the regulatory CpG dinucleotide can have far-reaching biological consequences.

Plant Villin Headpiece Domain Demonstrates a Novel Surface Charge Pattern and High Affinity for F-Actin.

Plants utilize multiple isoforms of villin, an F-actin regulating protein with an N-terminal gelsolin-like core and a distinct C-terminal headpiece domain. Unlike their vertebrate homologues, plant villins have a much longer linker polypeptide connecting the core and headpiece. Moreover, the linker-headpiece connection region in plant villins lacks sequence homology to the vertebrate villin sequences. It is unknown to what extent the plant villin headpiece structure and function resemble those of the well-studied vertebrate counterparts. Here we present the first solution NMR structure and backbone dynamics characterization of a headpiece from plants, villin isoform 4 from Arabidopsis thaliana. The villin 4 headpiece is a 63-residue domain (V4HP63) that adopts a typical headpiece fold with an aromatics core and a tryptophan-centered hydrophobic cap within its C-terminal subdomain. However, V4HP63 has a distinct N-terminal subdomain fold as well as a novel, high mobility loop due to the insertion of serine residue in the canonical sequence that follows the variable length loop in headpiece sequences. The domain binds actin filaments with micromolar affinity, like the vertebrate analogues. However, the V4HP63 surface charge pattern is novel and lacks certain features previously thought necessary for high-affinity F-actin binding. Utilizing the updated criteria for strong F-actin binding, we predict that the headpiece domains of all other villin isoforms in A. thaliana have high affinity for F-actin.

8-Oxoguanine Affects DNA Backbone Conformation in the EcoRI Recognition Site and Inhibits Its Cleavage by the Enzyme.

8-oxoguanine is one of the most abundant and impactful oxidative DNA lesions. However, the reasons underlying its effects, especially those not directly explained by the altered base pairing ability, are poorly understood. We report the effect of the lesion on the action of EcoRI, a widely used restriction endonuclease. Introduction of 8-oxoguanine inside, or adjacent to, the GAATTC recognition site embedded within the Drew-Dickerson dodecamer sequence notably reduced the EcoRI activity. Solution NMR revealed that 8-oxoguanine in the DNA duplex causes substantial alterations in the sugar-phosphate backbone conformation, inducing a BI→BII transition. Moreover, molecular dynamics of the complex suggested that 8-oxoguanine, although does not disrupt the sequence-specific contacts formed by the enzyme with DNA, shifts the distribution of BI/BII backbone conformers. Based on our data, we propose that the disruption of enzymatic cleavage can be linked with the altered backbone conformation and dynamics in the free oxidized DNA substrate and, possibly, at the protein-DNA interface.

Gelsolin-like activation of villin: calcium sensitivity of the long helix in domain 6.

Villin is a gelsolin-like cytoskeleton regulator localized in the brush border at the apical end of epithelial cells. Villin regulates microvilli by bundling F-actin at low calcium levels and severing it at high calcium levels. The villin polypeptide consists of six gelsolin-like repeats (V1-V6) and the unique, actin binding C-terminal headpiece domain (HP). Villin modular fragment V6-HP requires calcium to stay monomeric and bundle F-actin. Our data show that isolated V6 is monomeric and does not bind F-actin at any level of calcium. We propose that the 40-residue unfolded V6-to-HP linker can be a key regulatory element in villin's functions such as its interactions with F-actin. Here we report a calcium-bound solution nuclear magnetic resonance (NMR) structure of V6, which has a gelsolin-like fold with the long α-helix in the extended conformation. Intrinsic tryptophan fluorescence quenching reveals two-Kd calcium binding in V6 (Kd1 of 22 µM and Kd2 of 2.8 mM). According to our NMR data, the conformation of V6 responds the most to micromolar calcium. We show that the long α-helix and the adjacent residues form the calcium-sensitive elements in V6. These observations are consistent with the calcium activation of F-actin severing by villin analogous to the gelsolin helix-straightening mechanism.

Excision of 8-oxoguanine from methylated CpG dinucleotides by human 8-oxoguanine DNA glycosylase.

CpG dinucleotides are targets for epigenetic methylation, many of them bearing 5-methylcytosine (mCyt) in the human genome. Guanine in this context can be easily oxidized to 8-oxoguanine (oxoGua), which is repaired by 8-oxoguanine-DNA glycosylase (OGG1). We have studied how methylation affects the efficiency of oxoGua excision from damaged CpG dinucleotides. Methylation of the adjacent cytosine moderately decreased the oxoGua excision rate while methylation opposite oxoGua lowered the rate of product release. Cytosine methylation abolished stimulation of OGG1 by repair endonuclease APEX1. The OGG1 S326C polymorphic variant associated with lung cancer showed poorer base excision and lost sensitivity to the opposite-base methylation. The overall repair in the system reconstituted from purified proteins decreased for CpG with mCyt in the damaged strand.

An N-terminal, 830 residues intrinsically disordered region of the cytoskeleton-regulatory protein supervillin contains Myosin II- and F-actin-binding sites.

Supervillin, the largest member of the villin/gelsolin family, is a cytoskeleton regulating, peripheral membrane protein. Supervillin increases cell motility and promotes invasive activity in tumors. Major cytoskeletal interactors, including filamentous actin and myosin II, bind within the unique supervillin amino terminus, amino acids 1-830. The structural features of this key region of the supervillin polypeptide are unknown. Here, we utilize circular dichroism and bioinformatics sequence analysis to demonstrate that the N-terminal part of supervillin forms an extended intrinsically disordered region (IDR). Our combined data indicate that the N-terminus of human and bovine supervillin sequences (positions 1-830) represents an IDR, which is the largest IDR known to date in the villin/gelsolin family. Moreover, this result suggests a potentially novel mechanism of regulation of myosin II and F-actin via the intrinsically disordered N-terminal region of hub protein supervillin.

The isolated sixth gelsolin repeat and headpiece domain of villin bundle F-actin in the presence of calcium and are linked by a 40-residue unstructured sequence.

Villin is an F-actin regulating, modular protein with a gelsolin-like core and a distinct C-terminal "headpiece" domain. Localized in the microvilli of the absorptive epithelium, villin can bundle F-actin and, at higher calcium concentrations, is capable of a gelsolin-like F-actin severing. The headpiece domain can, in isolation, bind F-actin and is crucial for F-actin bundling by villin. While the three-dimensional structure of the isolated headpiece is known, its conformation in the context of attachment to the villin core remains unexplored. Furthermore, the dynamics of the linkage of the headpiece to the core has not been determined. To address these issues, we employ a 208-residue modular fragment of villin, D6-HP, which consists of the sixth gelsolin-like domain of villin (D6) and the headpiece (HP). We demonstrate that this protein fragment requires calcium for structural stability and, surprisingly, is capable of Ca2+-dependent F-actin bundling, suggesting that D6 contains a cryptic F-actin binding site. NMR resonance assignments and 15N relaxation measurements of D6-HP in 5 mM Ca2+ demonstrate that D6-HP consists of two independent structural domains (D6 and HP) connected by an unfolded 40-residue linker sequence. The headpiece domain in D6-HP retains its structure and interacts with D6 only through the linker sequence without engaging in other interactions. Chemical shift values indicate essentially the same secondary structure elements for D6 in D6-HP as in the highly homologous gelsolin domain 6. Thus, the headpiece domain of villin is structurally and functionally independent of the core domain.