Observation of the glycines in elastin using (13)C and (15)N solid-state NMR spectroscopy and isotopic labeling

A Two-State Model Describes the Temperature-Dependent Conformational Equilibrium in the Alanine-Rich Domains in Elastin

Elastin is the insoluble elastomeric protein that provides extensibility and resilience to vertebrate tissues. Limited high-resolution structural data for elastin are notably complex. To access this information, this protein is considered in the simplified context of its two general domain types, that is, hydrophobic (HP) and crosslinking (CL). The question of elastin's structure-function has directed the focus of nearly all previous studies in the literature to the unique repeating sequences characteristic of this protein, found primarily in the HP domains. The CL domains were assumed to play a very limited role in biological elasticity due in part to the significant α-helical character that was (incorrectly) predicted for these regions. In this study, the conformational heterogeneity of alanines in native elastin's CL domains is examined in the context of helix-coil transition theory (HCTT) using solid-state nuclear magnetic resonance (SSNMR) spectroscopy in tandem with strategic isotopic labeling. Helix and coil populations are observed at all temperatures, but the former increases significantly at lower temperatures. Below the glass transition temperature (T_g), two major populations of alanines in the CL regions are resolved by two-dimensional SSNMR; one-dimensional methods are used for characterization in nativelike conditions. The spectra of ¹³CO-Ala in the CL regions are simulated using an HCTT-based statistical mechanical representation. Below T_g, longer segments with significant helical probabilities are consistent with the experimental data. At higher temperatures, the SSNMR lineshapes are best fit with a distribution of shorter (Ala)_n segments, most in random coil. These results are used to refine a structure-function model for elastin in the context of HCTT, redirecting attention to the CL domains and their role in elasticity.

Efficient and accurate determination of 13C T1 and T1ρ relaxation time constants of high-mobility polymers from limited-resolution spectra with optimized pulse sequences and data fitting

The pulse sequences for the measurement of ¹³C T₁ and T_1ρ relaxation time constants in soft organic solids were modified to include steady-state nuclear Overhauser enhancement with direct polarization (ssNOE/DP). The increased signal intensities with ssNOE are particularly well-suited for highly mobile and structurally heterogeneous polymers and proteins like elastin. The phase cycling of these experiments was modified to yield datasets that require less time, with more accurate results. The "3D-fitting process" was developed and then optimized for natural-abundance ¹³C spectra of elastin, with its characteristic and significant overlap. A comparison of 3D-fitting with the similarly purposed SPORT (Geppi and Forte, 1999) illustrates that the former is more robust, with smaller uncertainty values and higher precision.

Paradoxes and wonders of intrinsic disorder: Stability of instability

This article continues a series of short comments on the paradoxes and wonders of the protein intrinsic disorder phenomenon by introducing the "stability of instability" paradox. Intrinsically disordered proteins (IDPs) are characterized by the lack of stable 3D-structure, and, as a result, have an exceptional ability to sustain exposure to extremely harsh environmental conditions (an illustration of the "you cannot break what is already broken" principle). Extended IDPs are known to possess extreme thermal and acid stability and are able either to keep their functionality under these extreme conditions or to rapidly regain their functionality after returning to the normal conditions. Furthermore, sturdiness of intrinsic disorder and its capability to "ignore" harsh conditions provides some interesting and important advantages to its carriers, at the molecular (e.g., the cell wall-anchored accumulation-associated protein playing a crucial role in intercellular adhesion within the biofilm of Staphylococcus epidermidis), supramolecular (e.g., protein complexes, biologic liquid-liquid phase transitions, and proteinaceous membrane-less organelles), and organismal levels (e.g., the recently popularized case of the microscopic animals, tardigrades, or water bears, that use intrinsically disordered proteins to survive desiccation).

13C, 2h NMR studies of structural and dynamical modifications of glucose-exposed porcine aortic elastin

Elastin, the principal component of the elastic fiber of the extracellular matrix, imparts to vertebrate tissues remarkable resilience and longevity. This work focuses on elucidating dynamical and structural modifications of porcine aortic elastin exposed to glucose by solid-state NMR spectroscopic and relaxation methodologies. Results from macroscopic stress-strain tests are also presented and indicate that glucose-treated elastin is mechanically stiffer than the same tissue without glucose treatment. These measurements show a large hysteresis in the stress-strain behavior of glucose-treated elastin-a well-known signature of viscoelasticity. Two-dimensional relaxation NMR methods were used to investigate the correlation time, distribution, and population of water in these samples. Differences are observed between the relative populations of water, whereas the measured correlation times of tumbling motion of water across the samples were similar. (13)C magic-angle-spinning NMR methods were applied to investigate structural and dynamical modifications after glucose treatment. Although some overall structure is preserved, the process of glucose exposure results in more heterogeneous structures and slower mobility. The correlation times of tumbling motion of the (13)C-(1)H internuclear vectors in the glucose-treated sample are larger than in untreated samples, pointing to their more rigid structure. The (13)C cross-polarization spectra reveal a notably increased α-helical character in the alanine motifs after glucose exposure. Results from molecular dynamics simulations are provided that add further insight into dynamical and structural changes of a short repeat, [VPGVG]5, an alanine pentamer, desmosine, and isodesmosine sites with and without glucose. The simulations point to changes in the entropic and energetic contributions in the retractive forces of VPGVG and AAAAA motifs. The most notable change is the increase of the energetic contribution in the retractive force due to peptide-glucose interactions of the VPGVG motif, which may play an important role in the observed stiffening in glucose-treated elastin.

Proline periodicity modulates the self-assembly properties of elastin-like polypeptides

Elastin is a self-assembling protein of the extracellular matrix that provides tissues with elastic extensibility and recoil. The monomeric precursor, tropoelastin, is highly hydrophobic yet remains substantially disordered and flexible in solution, due in large part to a high combined threshold of proline and glycine residues within hydrophobic sequences. In fact, proline-poor elastin-like sequences are known to form amyloid-like fibrils, rich in β-structure, from solution. On this basis, it is clear that hydrophobic elastin sequences are in general optimized to avoid an amyloid fate. However, a small number of hydrophobic domains near the C terminus of tropoelastin are substantially depleted of proline residues. Here we investigated the specific contribution of proline number and spacing to the structure and self-assembly propensities of elastin-like polypeptides. Increasing the spacing between proline residues significantly decreased the ability of polypeptides to reversibly self-associate. Real-time imaging of the assembly process revealed the presence of smaller colloidal droplets that displayed enhanced propensity to cluster into dense networks. Structural characterization showed that these aggregates were enriched in β-structure but unable to bind thioflavin-T. These data strongly support a model where proline-poor regions of the elastin monomer provide a unique contribution to assembly and suggest a role for localized β-sheet in mediating self-assembly interactions.

Structural disorder and dynamics of elastin

Elastin is a self-assembling, extracellular-matrix protein that is the major provider of tissue elasticity. Here we review structural studies of elastin from over four decades, and draw together evidence for solution flexibility and conformational disorder that is inherent in all levels of structural organization. The characterization of disorder is consistent with an entropy-driven mechanism of elastic recoil. We conclude that conformational disorder is a constitutive feature of elastin structure and function.

Circular dichroism and UV-resonance Raman investigation of the temperature dependence of the conformations of linear and cyclic elastin

We used electronic circular dichroism (CD) and UV resonance Raman (UVRR) spectroscopy at 204 nm excitation to examine the temperature dependence of conformational changes in cyclic and linear elastin peptides. We utilize CD spectroscopy to study global conformation changes in elastin peptides, while UVRR is utilized to probe the local conformation and hydrogen bonding of Val and Pro peptide bonds. Our results indicate that at 20 degrees C cyclic elastin predominantly populates distorted beta-strand, beta-type II and beta-type III turn conformations. At 60 degrees C, the beta-type II turn population increases, while the distorted beta-strand population decreases. Linear elastin predominantly adopts distorted beta-strand and beta-type III turn conformations with some beta-type II turn population at 20 degrees C. Increasing temperature to 60 degrees C results in a small increase in the turn population.

Structural insights into the elastin mimetic (LGGVG)6 using solid-state 13C NMR experiments and statistical analysis of the PDB

Elastin is a crosslinked hydrophobic protein found in abundance in vertebrate tissue and is the source of elasticity in connective tissues and blood vessels. The repeating polypeptide sequences found in the hydrophobic domains of elastin have been the focus of many studies that attempt to understand the function of the native protein on a molecular scale. In this study, the central residues of the (LGGVG)(6) elastin mimetic are targeted. Using a combination of a statistical analysis based on structures in the Brookhaven Protein Data Bank (PDB), 1D cross-polarization magic-angle-spinning (CPMAS) NMR spectroscopy, and 2D off-magic-angle-spinning (OMAS) spin-diffusion experiments, it is determined that none of the residues are found in a singular regular, highly ordered structure. Instead, like the poly(VPGVG) elastin mimetics, there are multiple conformations and significant disorder. Furthermore, the conformational ensembles are not reflective of proteins generally, as in the PDB, suggesting that the structure distributions in elastin mimetics are unique to these peptides and are a salient feature of the functional model of the native protein.

Flexibility in the solution structure of human tropoelastin

We investigated the flexibility of full-length tropoelastin in solution by using far- and near-ultraviolet circular dichroism (UV CD) and fluorescence spectroscopy to probe for structural flexibility and residue mobility within secondary and tertiary features of the monomer. Fluorescence spectroscopy revealed the presence of exposed hydrophobicity through the binding of the hydrophobic probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate (bis-ANS), which demonstrates that hydrophobic regions form clusters and are not confined to a molecular core. Near-UV CD indicated substantial mobility of aromatic residues. Structural prediction programs (PONDR, DisEMBL, and Globplot version 2.0) estimated 75 +/- 2% disorder in the tertiary structure of tropoelastin on the basis of primary sequence information. A single-site substitution of Trp for Gln (Q513W) at the tropoelastin domain 25-26 interface facilitated fluorescence spectroscopy for revealing that this region is exposed to solvent. Polarization anisotropy demonstrated substantial flexibility of W513 and little change upon denaturation of the monomer with guanidine hydrochloride. Comparable movement was found for native sequence aromatic residues in the presence of glycosaminoglycans and trifluoroethanol. These data prove the intrinsic flexibility of specific residues and adjacent sequences in any native conformation(s) they may take. This study is the first characterization of the level of mobility in defined regions of the full-length tropoelastin monomer and provides direct evidence for regions of flexible structure in tropoelastin.

Heterogeneity in the Conformation of Valine in the Elastin Mimetic (LGGVG)6 as Shown by Solid-State 13C NMR Spectroscopy

Elastin is an abundant protein found in vertebrates and is the source of elasticity in connective tissues and blood vessels. The repeating polypeptide sequences found in the hydrophobic domains of elastin have been the focus of many studies that attempt to understand the function of the native protein on a molecular scale. In this communication, the (LGGVG)6 elastin mimetic is characterized by solid-state 13C NMR spectroscopy. Through the use of a combination of a statistical analysis based on the Protein Data Bank, one-dimensional cross-polarization magic-angle-spinning NMR spectroscopy, and two-dimensional off-magic-angle-spinning spin-diffusion experiments, it is determined that this tandem repeat does not form a regular, highly ordered structure. Instead, like the poly(VPGVG) elastin mimetics, the valine has a twofold heterogeneity, although the conformations of these two populations differ from one peptide to the other.

Cooperativity between the Hydrophobic and Cross-linking Domains of Elastin

The principal protein component of the elastic fiber found in elastic tissues is elastin, an amorphous, cross-linked biopolymer that is assembled from a high molecular weight monomer. The hydrophobic and cross-linking domains of elastin have been considered separate and independent, such that changes to one region are not thought to affect the other. However, results from these solid-state 13C NMR experiments demonstrate that cooperativity in protein folding exists between the two domain types. The sequence of the EP20-24-24 polypeptide has three hydrophobic sequences from exons 20 and 24 of the soluble monomer tropoelastin, interspersed with cross-linking domains constructed from exons 21 and 23. In the middle of each cross-linking domain is a "hinge" sequence. When this pentapeptide is replaced with alanines, as in EP20-24-24[23U], its properties are changed. In addition to the expected increase in alpha-helical content and the resulting increase in rigidity of the cross-linking domains, changes to the organization of the hydrophobic regions are also observed. Using one-dimensional CPMAS (cross-polarization with magic angle spinning) techniques, including spectral editing and relaxation measurements, evidence for a change in dynamics to both domain types is observed. Furthermore, it is likely that the methyl groups of the leucines of the hydrophobic domains are also affected by the substitution to the hinge region of the cross-linking sequences. This cooperativity between the two domain types brings new questions to the phenomenon of coacervation in elastin polypeptides and strongly suggests that functional models for the protein must include a role for the cross-linking regions.

Inverse Temperature Transition of a Biomimetic Elastin Model: Reactive Flux Analysis of Folding/Unfolding and Its Coupling to Solvent Dielectric Relaxation

The inverse temperature transition (ITT) of a biomimetic model for elastin, capped GVG(VPGVG) in liquid water, is investigated by a comprehensive classical molecular dynamics study. The temperature dependence of the solvation structure and dynamics of the octapeptide are compared using three common force fields, CHARMM, GROMOS, and OPLS. While these force fields differ in quantitative detail, they all predict this octapeptide to undergo a "folding transition" to closed conformations upon heating and a subsequent "unfolding transition" to open conformations at still higher temperatures, thus reproducing the ITT scenario. The peptide kinetics is analyzed within the reactive flux formalism applied to the largest-amplitude mode extracted from principal component analysis, and the solvent's dielectric fluctuations are obtained from the total water dipole autocorrelations. Most importantly, preliminary evidence for an intimate coupling of peptide folding/unfolding dynamics, and thus the ITT, and dielectric relaxation of bulk water is given, possibly being consistent with a "slave mode" picture.

Folding and unfolding of an elastinlike oligopeptide: "inverse temperature transition," reentrance, and hydrogen-bond dynamics

The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy.

Temperature-dependent conformational transitions and hydrogen-bond dynamics of the elastin-like octapeptide GVG(VPGVG): a molecular-dynamics study

A joint experimental/theoretical investigation of the elastin-like octapeptide GVG(VPGVG) was carried out. In this article a comprehensive molecular-dynamics study of the temperature-dependent folding and unfolding of the octapeptide is presented. The current study, as well as its experimental counterpart (see companion article in this issue) find that this peptide undergoes an inverse temperature transition (ITT), leading to a folding at approximately 40-60 degrees C. In addition, an unfolding transition is identified at unusually high temperatures approaching the normal boiling point of water. Due to the small size of the system, two broad temperature regimes are found: the ITT regime at approximately 10-60 degrees C and the unfolding regime at approximately T > 60 degrees C, where the peptide has a maximum probability of being folded at T approximately 60 degrees C. A detailed molecular picture involving a thermodynamic order parameter, or reaction coordinate, for this process is presented along with a time-correlation function analysis of the hydrogen-bond dynamics within the peptide as well as between the peptide and solvating water molecules. Correlation with experimental evidence and ramifications on the properties of elastin are discussed.

13C CPMAS NMR studies of the elastin-like polypeptide (LGGVG)n

The elucidation of structure-function relationships in insoluble elastin is often approached using elastin-like polypeptides. In this manner, the characterization of the different regions in this extensive biopolymer may be facilitated in a "piece-wise" manner. Our solid-state NMR experiments indicate that (LGGVG)n has structural similarities to elastin and some elastin peptides, providing support for the utility of the mimetic peptides. Furthermore, previous NMR and CD studies indicated that the structure of the elastin-like polypeptide (LGGVG)n in solution is best described as a "conformational ensemble" with a mixture of type I and II beta-turns, in addition to unfolded regions. Our data indicate that the peptide does not adopt a single conformation in the solid state, lending further support to models for elastin that involve significant conformational heterogeneity.