Scanning microcalorimetry and circular dichroism study of melting of the natural polypeptides in the left-handed helical conformation

Phosphorylation Increases Persistence Length and End-to-End Distance of a Segment of Tau Protein

Intrinsically disordered regions of proteins, which lack unique tertiary structure under physiological conditions, are enriched in phosphorylation sites and in significant local bias toward the polyproline II conformation. The overrepresented coincidence of this posttranslational regulatory signal and local conformational bias within unstructured regions raises a question: can phosphorylation serve to manipulate the conformational preferences of a disordered protein? In this study, we use time-resolved fluorescence resonance energy transfer and a, to our knowledge, novel data analysis method to directly measure the end-to-end distance distribution of a phosphorylatable peptide derived from the human microtubule associated protein tau. Our results show that phosphorylation at threonine or serine extends the end-to-end distance and increases the effective persistence length of the tested model peptides. Unexpectedly, the extension is independent of salt concentration, suggestive of a nonelectrostatic origin. The phosphorylation extension and stiffening effect provides a peptide-scale physical interpretation for the posttranslational regulation of the highly abundant protein-protein interactions found in disordered proteins, as well as a potential insight into the regulatory mechanism of the tau protein's microtubule binding activity.

Contiguous O-galactosylation of 4(R)-hydroxy-l-proline residues forms very stable polyproline II helices

The hydroxyproline-rich glycoproteins (HRGPs) are the major structural proteins of the extracellular matrix of algae and land plants. They are characterized by a rigid polyproline type II (PPII) conformation and extensive O-glycosylation of 4(R)-hydroxy-l-proline (Hyp) residues, which is a unique post-translational modification of proteins. The functional consequences of HRGP glycosylation remains unclear, but they have been implicated in contributing to their structural rigidity. Here, we have investigated the effects of naturally occurring beta-O-galactosylation of Hyp residues on the conformational stability of the PPII helix. In a series of well-defined model peptides Ac-(l-proline)(9)-NH(2) (1), Ac-(Hyp)(9)-NH(2) (2), and Ac-[Hyp(beta-d-galactose)](9)-NH(2) (3) we demonstrate that contiguous O-glycosylation of Hyp residues causes a dramatic increase in the thermal stability of the PPII helix according to analysis of thermal melting curves. This represents the first quantitative data on the contributions of glycosylation to stabilizing the PPII conformation. Molecular modeling indicates the increase in conformational stability may be due to a regular network of interglycan and glycan-peptide hydrogen bonds, in which the carbohydrate residues form a hydrophilic "overcoat" of the PPII helix. Evidence of this shielding effect of the amide backbone may be provided by analysis of the circular dichroism bands, which indicates an increase in the rho value of 3 relative to 1 and 2. This study gives further insight into the effects of naturally occurring Hyp beta-O-linked glycans on the PPII conformation as found in HRGPs in plant cell walls and also indicates that polyproline sequences may be suitable for the development of molecular scaffolds for the presentation of glycan structures.

A (4R)- or a (4S)-Fluoroproline Residue in Position Xaa of the (Xaa-Yaa-Gly) Collagen Repeat Severely Affects Triple-Helix Formation

The triple-helical fold of collagen requires the presence of a glycine residue at every third position in the peptide sequence and is stabilized by proline and (4R)-4-hydroxyproline residues in positions Xaa and Yaa of the (Xaa-Yaa-Gly) triplets, respectively. Regular down/up puckering of these Xaa/Yaa residues is possibly responsible for the tight packing of the three peptide strands, which have a polyproline-II-like structure, into the supercoiled helix. (4R)-Configured electronegative substituents such as a hydroxy group or a fluorine substituent on the pyrrolidine ring of the residue in the Yaa position favor the up pucker and thus significantly stabilize the triple helix. A similar effect was expected from the corresponding (4S)-isomers in the Xaa positions, but the opposite effect has been observed with (4S)-hydroxyproline, a result that has been speculatively attributed to steric effects. In this study, (4R)- and (4S)-fluoroproline residues were introduced into the Xaa position and potential steric effects were thus avoided. Contrary to expectations, (4S)-fluoroproline prevents triple-helix formation, whereas (4R)-fluoroproline stabilizes the polyPro II conformation, but without supercoiling of the three strands. The latter observation suggests that folding of the single chains into a polyproline II helix is not directly associated with triple helix formation and that fine tuning of van der Waals contacts, electrostatic interactions, and stereoelectronic effects is required for optimal packing into a triple helix.

The chain register in heterotrimeric collagen peptides affects triple helix stability and folding kinetics

Collagen type IV is a highly specialized form of collagen found only in basement membranes, where it provides mechanical stability and structural integrity to tissues and organs, and binding sites for cell adhesion. In its ubiquitous form, collagen type IV consists of two alpha1 chains and one alpha2 chain, whose internal alignment within the triple helix seems to exert a strong influence on the binding affinity to alpha1beta1 integrin receptor. This has been assessed recently using two synthetic collagen peptides that contain the cell adhesion epitope of collagen type IV and are assembled into the most plausible alpha1alpha2alpha1' and alpha2alpha1alpha1' registers. In the present study, the effects of the chain register on the stability of the triple helix and the folding kinetics of these collagen peptides were investigated by CD spectroscopy and microcalorimetry. The results revealed a multi-domain structural organization for both trimers, with an unexpected strong effect of the chain alignment on the conformational stability. Molecular dynamics simulations served to rationalize more properly the experimental results.

Structural properties of a collagenous heterotrimer that mimics the collagenase cleavage site of collagen type I

Collagens contain sequence- and conformation-dependent epitopes responsible for their digestion by collagenases at specific loci. A synthetic heterotrimer construct containing the collagenase cleavage site of collagen type I was found to mimic perfectly native collagen in terms of selectivity and mode of enzymatic degradation. The NMR conformational analysis of this molecule clearly revealed the presence of two structural domains, i.e. a triple helix spanning the Gly-Pro-Hyp repeats and a less ordered portion corresponding to the collagenase cleavage site where the three chains are aligned in extended conformation with loose interchain contacts. These structural properties allow for additional insights into the very particular mechanism of collagen digestion by collagenases.

Structural organization of the fibrin(ogen) alpha C-domain

We hypothesized that the alpha C-domain of human fibrinogen (residues hA alpha 221-610) and of other species consists of a compact COOH-terminal region (hA alpha 392-610) and a flexible NH(2)-terminal connector region (hA alpha 221-391) which may contain some regular structure [Weisel and Medved (2001) Ann. N.Y. Acad. Sci. 936, 312-327]. To test this hypothesis, we expressed in E. coli recombinant fragments corresponding to the full-length human alpha C-domain and its NH(2)- and COOH-terminal regions as well as their bovine counterparts, bA alpha 224-568, bA alpha 224-373, and bA alpha 374-568(538), respectively, and tested their folding status by fluorescence spectroscopy, circular dichroism (CD), and differential scanning calorimetry (DSC). All three methods revealed heat-induced unfolding transitions in the full-length bA alpha 224-568 and its two COOH-terminal fragments, indicating that the COOH-terminal portion of the bovine alpha C-domain is folded into a compact cooperative structure. Similar results were obtained by CD and DSC with the full-length and the COOH-terminal h392-610 human fragments. The NH(2)-terminal fragments of both species, b224-373 and h221-392, did not exhibit any sign of a compact structure. However, their heat capacity functions, CD spectra, and temperature dependence of ellipticity at 222 nm were typical for peptides in the extended helical poly(L-proline) type II conformation (PPII), suggesting that they contain this type of regular structure. This is consistent with the presence of proline-rich tandem repeats in the sequence of both bovine and human connector regions. These results indicate that both bovine and human fibrinogen alpha C-domains consist of a compact globular cooperative unit attached to the bulk of the molecule by an extended NH(2)-terminal connector region with a PPII conformation.

Synthesis of heterotrimeric collagen peptides containing the alpha1beta1 integrin recognition site of collagen type IV

Collagen type IV provides a biomechanically stable scaffold into which the other constituents of basement membranes are incorporated, but it also plays an important role in cell adhesion. This occurs with collagen type IV mainly via the alpha1beta1 integrin, and the proposed epitope involved in this type of collagen/integrin interaction corresponds to a non-sequential R/Xaa/D motif, where the arginine and aspartate residues are provided by the alpha2 and alpha1 chains of the collagen molecule, respectively. Since the stagger of the three alpha chains in native collagen type IV is still unknown and different alignments of the chains lead to different spatial epitopes, two heterotrimeric collagen peptides containing the natural 457-469 sequences of the cell adhesion site were synthesized in which the single chains were assembled via disulfide bonds into the two most plausible alpha1alpha2alpha1' and alpha2alpha1alpha1' registers. The differentiated triple-helical stabilities of the two heterotrimers suggest a significant structural role of the chain register in collagen, although the binding to alpha1beta1 integrin is apparently less affected as indicated by preliminary experiments.

Extreme stability of helices formed by water-soluble poly-N-substituted glycines (polypeptoids) with alpha-chiral side chains

Poly-N-substituted glycines or "peptoids" are protease-stable peptide mimics. Although the peptoid backbone is achiral and lacks hydrogen-bond donors, substitution with alpha-chiral side chains can drive the formation of stable helices that give rise to intense CD spectra. To systematically study the solution properties and stability of water-soluble peptoid helices with alpha-chiral side chains, we have synthesized and characterized an amphipathic, 36-residue N-substituted glycine oligomer. CD was used to investigate effects of concentration and solvent environment on this helical peptoid. We saw no significant dependence of helical structure on concentration. Intense, "alpha-helix-like" CD spectra were observed for the 36-mer in aqueous, 2,2,2-trifluorethanol (TFE), and methanol solution, proving a relative insensitivity of peptoid helical structure to solvent environment. While CD spectra taken in these different solvents were fundamentally similar in shape, we did observe some interesting differences in the intensities of particular CD bands in the various solvents. For example, the addition of TFE to an aqueous solvent increases the degree of peptoid helicity, as is observed for polypeptide alpha-helices. Moreover, the helical structure of peptoids appears to be virtually unaffected by heat, even in an aqueous buffer containing 8 M urea. The extraordinary resistance of these peptoid helices to denaturation is consistent with a dominant role of steric forces in their structural stabilization. The structured polypeptoids studied here may have potential as robust mimics of helical polypeptides of therapeutic interest.

Is polyproline II helix the killer conformation? A Raman optical activity study of the amyloidogenic prefibrillar intermediate of human lysozyme

The amyloidogenic prefibrillar partially denatured intermediate of human lysozyme, prepared by heating the native protein to 57 degrees C at pH 2.0, was studied using Raman optical activity (ROA). A positive band in the room temperature ROA spectrum of the native protein at approximately 1345 cm(-1), assigned to a hydrated form of alpha-helix, is not present in that of the prefibrillar intermediate, where a new strong positive band at approximately 1318 cm(-1) appears instead that is assigned to the poly(l-proline) II (PPII)-helical conformation. A sharp negative band at approximately 1241 cm(-1) in the native protein, assigned to beta-strand, shows little change in the ROA spectrum of the prefibrillar intermediate. The disappearance of a positive ROA band at approximately 1551 cm(-1) assigned to vibrations of tryptophan side-chains indicates that major conformational changes have occurred among the five tryptophan residues present in human lysozyme, four of which are located in the alpha-domain. The various ROA data suggest that a substantial loss of tertiary structure has occurred in the prefibrillar intermediate and that this is located more in the alpha-domain than in the beta-domain. There is no evidence for any increase in beta-structure. The ROA spectrum of hen lysozyme, which does not form amyloid fibrils so readily, remains much more native-like on heating to 57 degrees C at pH 2.0. The thermal behaviour of the alanine-rich alpha-helical peptide AK21 in aqueous solution was found to be similar to that of human lysozyme. Hydrated alpha-helix therefore appears to readily undergo a conformational change to PPII structure on heating, which may be a key step in the conversion of alpha-helix into beta-sheet in the formation of amyloid fibrils in human lysozyme. Since it is extended, flexible, lacks intrachain hydrogen bonds and is fully hydrated in aqueous solution, PPII helix has the appropriate characteristics to be implicated as a critical conformational element in many conformational diseases. Disorder of the PPII type may be a sine qua non for the formation of regular fibrils; whereas the more dynamic disorder of the random coil may lead only to amorphous aggregates.