Evidence for small-molecule-mediated loop stabilization in the structure of the isolated Pin1 WW domain

Design of Functional Globular β-Sheet Miniproteins

The design of discrete β-sheet peptides is far less advanced than e. g. the design of α-helical peptides. The reputation of β-sheet peptides as being poorly soluble and aggregation-prone often hinders active design efforts. Here, we show that this reputation is unfounded. We demonstrate this by looking at the β-hairpin and WW domain. Their structure and folding have been extensively studied and they have long served as model systems to investigate protein folding and folding kinetics. The resulting fundamental understanding has led to the development of hyperstable β-sheet scaffolds that fold at temperatures of 100 °C or high concentrations of denaturants. These have been used to design functional miniproteins with protein or nucleic acid binding properties, in some cases with such success that medical applications are conceivable. The β-sheet scaffolds are not always completely rigid, but can be specifically designed to respond to changes in pH, redox potential or presence of metal ions. Some engineered β-sheet peptides also exhibit catalytic properties, although not comparable to those of natural proteins. Previous reviews have focused on the design of stably folded and non-aggregating β-sheet sequences. In our review, we now also address design strategies to obtain functional miniproteins from β-sheet folding motifs.

Identification of novel functional mini-receptors by combinatorial screening of split-WW domains

β-Sheet motifs such as the WW domain are increasingly being explored as building blocks for synthetic biological applications. Since the sequence-structure relationships of β-sheet motifs are generally complex compared to the well-studied α-helical coiled coil (CC), other approaches such as combinatorial screening should be included to vary the function of the peptide. In this study, we present a combinatorial approach to identify novel functional mini-proteins based on the WW-domain scaffold, which takes advantage of the successful reconstitution of the fragmented WW domain of hPin1 (hPin1WW) by CC association. Fragmentation of hPin1WW was performed in both loop 1 (CC-hPin1WW-L1) and loop 2 (CC-hPin1WW-L2), and the respective fragments were linked to the strands of an antiparallel heterodimeric CC. Structural analysis by CD and NMR spectroscopy revealed structural reconstitution of the WW-domain scaffold only in CC-hPin1WW-L1, but not in CC-hPin1WW-L2. Furthermore, by using 1H-15N HSQC NMR, fluorescence and CD spectroscopy, we demonstrated that binding properties of fragmented hPin1WW in CC-hPin1WW-L1 were fully restored by CC association. To demonstrate the power of this approach as a combinatorial screening platform, we synthesized a four-by-six library of N- and C-terminal hPin1WW-CC peptide fragments that was screened for a WW domain that preferentially binds to ATP over cAMP, phophocholine, or IP6. Using this screening platform, we identified one WW domain, which specifically binds ATP, and a phosphorylcholine-specific WW-based mini-receptor, both having binding dissociation constants in the lower micromolar range.

Evaluation of β-Amino Acid Replacements in Protein Loops: Effects on Conformational Stability and Structure

β-Amino acids have a backbone that is expanded by one carbon atom relative to α-amino acids, and β residues have been investigated as subunits in protein-like molecules that adopt discrete and predictable conformations. Two classes of β residue have been widely explored in the context of generating α-helix-like conformations: β³ -amino acids, which are homologous to α-amino acids and bear a side chain on the backbone carbon adjacent to nitrogen, and residues constrained by a five-membered ring, such the one derived from trans-2-aminocyclopentanecarboxylic acid (ACPC). Substitution of α residues with their β³  homologues within an α-helix-forming sequence generally causes a decrease in conformational stability. Use of a ring-constrained β residue, however, can offset the destabilizing effect of α→β substitution. Here we extend the study of α→β substitutions, involving both β³ and ACPC residues, to short loops within a small tertiary motif. We start from previously reported variants of the Pin1 WW domain that contain a two-, three-, or four-residue β-hairpin loop, and we evaluate α→β replacements at each loop position for each variant. By referral to the ϕ,ψ angles of the native structure, one can choose a stereochemically appropriate ACPC residue. Use of such logically chosen ACPC residues enhances conformational stability in several cases. Crystal structures of three β-containing Pin1 WW domain variants show that a native-like tertiary structure is maintained in each case.

The Dependence of Carbohydrate-Aromatic Interaction Strengths on the Structure of the Carbohydrate

Interactions between proteins and carbohydrates are ubiquitous in biology. Therefore, understanding the factors that determine their affinity and selectivity are correspondingly important. Herein, we have determined the relative strengths of intramolecular interactions between a series of monosaccharides and an aromatic ring close to the glycosylation site in an N-glycoprotein host. We employed the enhanced aromatic sequon, a structural motif found in the reverse turns of some N-glycoproteins, to facilitate face-to-face monosaccharide-aromatic interactions. A protein host was used because the dependence of the folding energetics on the identity of the monosaccharide can be accurately measured to assess the strength of the carbohydrate-aromatic interaction. Our data demonstrate that the carbohydrate-aromatic interaction strengths are moderately affected by changes in the stereochemistry and identity of the substituents on the pyranose rings of the sugars. Galactose seems to make the weakest and allose the strongest sugar-aromatic interactions, with glucose, N-acetylglucosamine (GlcNAc) and mannose in between. The NMR solution structures of several of the monosaccharide-containing N-glycoproteins were solved to further understand the origins of the similarities and differences between the monosaccharide-aromatic interaction energies. Peracetylation of the monosaccharides substantially increases the strength of the sugar-aromatic interaction in the context of our N-glycoprotein host. Finally, we discuss our results in light of recent literature regarding the contribution of electrostatics to CH-π interactions and speculate on what our observations imply about the absolute conservation of GlcNAc as the monosaccharide through which N-linked glycans are attached to glycoproteins in eukaryotes.

Noncovalent binding of a cyclic peptide inhibitor to the peptidyl-prolyl isomerase Pin1, explored by hydrogen exchange mass spectrometry

Pin1 is a peptidyl-prolyl isomerase (PPIase) that plays a central role in eukaryotic cell cycle regulation, making this protein an interesting target for cancer therapy. Pin1 exhibits high specificity for substrates where proline is preceded by phosphoserine or phosphothreonine. The protein comprises an N-terminal WW (tryptophan–tryptophan) domain and a C-terminal PPIase domain. The cyclic peptide [CRYPEVEIC] (square brackets are used to denote the cyclic structure) represents a lead compound for a new class of nonphosphorylated Pin1 inhibitors. Unfortunately, it has not been possible thus far to characterize the Pin1–[CRYPEVEIC] complex by X-ray crystallography. Thus, the exact binding mode remains unknown. The current work employs hydrogen/deuterium exchange mass spectrometry for gaining insights into the Pin1–[CRYPEVEIC] interactions. The WW domain shows extensive conformational dynamics, both in the presence and in the absence of ligand. In contrast, profound changes in deuteration kinetics are observed in the PPIase domain after the addition of [CRYPEVEIC]. The secondary structure elements β2, α3, and α4 exhibit markedly reduced deuteration, consistent with their postulated involvement in ligand binding. Unexpectedly, [CRYPEVEIC] destabilizes the range of residues 61–86, a segment that comprises basic side chains that normally interact with the substrate phosphate. This destabilization is likely caused by steric clashes with Y3 or E5 of the inhibitor. Ligand-induced destabilization has previously been reported for a few other proteins, but effects of this type are not very common. Our findings suggest that future crystallization trials on Pin1 variants deleted for residues in the 61–86 range might provide a path towards high-resolution X-ray structures of Pin1 bound to cyclic peptide inhibitors.

Evidence for phenylalanine zipper-mediated dimerization in the X-ray crystal structure of a magainin 2 analogue

High-resolution structure elucidation has been challenging for the large group of host-defense peptides that form helices on or within membranes but do not manifest a strong folding propensity in aqueous solution. Here we report the crystal structure of an analogue of the widely studied host-defense peptide magainin 2. Magainin 2 (S8A, G13A, G18A) is a designed variant that displays enhanced antibacterial activity relative to the natural peptide. The crystal structure of magainin 2 (S8A, G13A, G18A), obtained for the racemic form, features a dimerization mode that has previously been proposed to play a role in the antibacterial activity of magainin 2 and related peptides.