Polycation-π Interactions Are a Driving Force for Molecular Recognition by an Intrinsically Disordered Oncoprotein Family

Molecular recognition by intrinsically disordered proteins (IDPs) commonly involves specific localized contacts and target-induced disorder to order transitions. However, some IDPs remain disordered in the bound state, a phenomenon coined "fuzziness", often characterized by IDP polyvalency, sequence-insensitivity and a dynamic ensemble of disordered bound-state conformations. Besides the above general features, specific biophysical models for fuzzy interactions are mostly lacking. The transcriptional activation domain of the Ewing's Sarcoma oncoprotein family (EAD) is an IDP that exhibits many features of fuzziness, with multiple EAD aromatic side chains driving molecular recognition. Considering the prevalent role of cation-π interactions at various protein-protein interfaces, we hypothesized that EAD-target binding involves polycation- π contacts between a disordered EAD and basic residues on the target. Herein we evaluated the polycation-π hypothesis via functional and theoretical interrogation of EAD variants. The experimental effects of a range of EAD sequence variations, including aromatic number, aromatic density and charge perturbations, all support the cation-π model. Moreover, the activity trends observed are well captured by a coarse-grained EAD chain model and a corresponding analytical model based on interaction between EAD aromatics and surface cations of a generic globular target. EAD-target binding, in the context of pathological Ewing's Sarcoma oncoproteins, is thus seen to be driven by a balance between EAD conformational entropy and favorable EAD-target cation-π contacts. Such a highly versatile mode of molecular recognition offers a general conceptual framework for promiscuous target recognition by polyvalent IDPs. © 2013 Song et al

Multi-Scaled Explorations of Binding-Induced Folding of Intrinsically Disordered Protein Inhibitor IA3 to its Target Enzyme

Biomolecular function is realized by recognition, and increasing evidence shows that recognition is determined not only by structure but also by flexibility and dynamics. We explored a biomolecular recognition process that involves a major conformational change – protein folding. In particular, we explore the binding-induced folding of IA3, an intrinsically disordered protein that blocks the active site cleft of the yeast aspartic proteinase saccharopepsin (YPrA) by folding its own N-terminal residues into an amphipathic alpha helix. We developed a multi-scaled approach that explores the underlying mechanism by combining structure-based molecular dynamics simulations at the residue level with a stochastic path method at the atomic level. Both the free energy profile and the associated kinetic paths reveal a common scheme whereby IA3 binds to its target enzyme prior to folding itself into a helix. This theoretical result is consistent with recent time-resolved experiments. Furthermore, exploration of the detailed trajectories reveals the important roles of non-native interactions in the initial binding that occurs prior to IA3 folding. In contrast to the common view that non-native interactions contribute only to the roughness of landscapes and impede binding, the non-native interactions here facilitate binding by reducing significantly the entropic search space in the landscape. The information gained from multi-scaled simulations of the folding of this intrinsically disordered protein in the presence of its binding target may prove useful in the design of novel inhibitors of aspartic proteinases

Protein species as diagnostic markers

Many diseases are associated with protein species perturbations. A prominent example of an established diagnostic marker is the glycated protein species of hemoglobin, termed HbA1c. HbA1c concentration is increased in the blood of diabetes mellitus patients due to their poor control of blood glucose levels resulting in an increased non-enzymatic glycosylation of hemoglobin producing HbA1c. This important diagnostic marker is routinely measured in the blood of diabetes patients. As in the case of HbA1c, protein species can mirror pathophysiological events. Shifts in the levels of protein species can be associated with or even be responsible for disease making them well suited as diagnostic markers. However, only a few protein species are currently used as diagnostic markers in routine clinical chemistry laboratories, despite being widely established in clinical proteomics research. This review provides an overview of the biochemical characteristics associated with protein species as well as examples of pathophysiological mechanisms, which cause modifications in the protein species composition, thereby emphasizing the importance of screening for protein markers at the species level. Further, we highlight techniques, which are currently utilized for investigating protein species markers in clinical research. Biological significance: The success rate of FDA approved diagnostic protein markers until today is very low compared to the number of published candidate disease markers. It is hypothesized that one important reason is the gene-centric view which is still followed in clinical proteomics: In many investigations proteins are still digested in small peptides thus making it nearly impossible to discriminate between healthy proteins and pathologic proteins causing diseases. Thus this review is focusing on the biochemistry and patho-biochemistry of proteins, is highlighting the need for screening for disease markers on the protein species level and is giving an overview about available techniques

Crystallography on a chip

A new chip-based crystal-mounting approach for rapid room-temperature data collection from numerous crystals is described. This work was motivated by the recent development of X-ray free-electron lasers. These novel sources deliver very intense femtosecond X-ray pulses that promise to yield high-resolution diffraction data of nanocrystals before their destruction by radiation damage. Thus, the concept of `diffraction before destruction' requires rapid replenishment of the sample for each exposure. The chip promotes the self-assembly of an array of protein crystals on a surface. Rough features on the surface cause the crystals to adopt random orientations, allowing efficient sampling of reciprocal space

Structure of an intermediate state in protein folding and aggregation

Protein-folding intermediates have been implicated in amyloid fibril formation involved in neurodegenerative disorders. However, the structural mechanisms by which intermediates initiate fibrillar aggregation have remained largely elusive. To gain insight, we used relaxation dispersion nuclear magnetic resonance spectroscopy to determine the structure of a low-populated, on-pathway folding intermediate of the A39V/N53P/V55L (A, Ala; V, Val; N, Asn; P, Pro; L, Leu) Fyn SH3 domain. The carboxyl terminus remains disordered in this intermediate, thereby exposing the aggregation-prone amino-terminal β strand. Accordingly, mutants lacking the carboxyl terminus and thus mimicking the intermediate fail to safeguard the folding route and spontaneously form fibrillar aggregates. The structure provides a detailed characterization of the non-native interactions stabilizing an aggregation-prone intermediate under native conditions and insight into how such an intermediate can derail folding and initiate fibrillation

Structure of a Transient Intermediate at the Edge between Folding and Aggregation into Amyloid Fibrils from NMR Relaxation Dispersion Experiments

Protein folding intermediates are implicated in amyloid fibril formation but difficult to characterize. CPMG NMR relaxation dispersion experiments allowed us to detect a previously unknown intermediate on the folding pathway of the Fyn SH3 A39V/N53P/V55L and to reconstruct chemical shifts and RDCs/RCSAs for this 2% populated intermediate. Calculation of the high-resolution structure of the “invisible” intermediate from these experimental restraints using the Camshift strategy revealed a native-like arrangement of 4 of the 5 native beta-strands stabilized by several non-native long-range interactions. By contrast, the C-terminus remains disordered, leaving an aggregation-prone strand exposed. Accordingly, mutants mimicking this intermediate spontaneously form amyloid fibrils. This structure provides a detailed picture of how an intermediate can facilitate both, folding but also misfolding/aggregation

NMR Solution Structure of an Invisible Protein State at the Edge between Folding and Aggregation into Amyloid Fibrils

Protein folding intermediates have been implicated in amyloid fibril formation involved in neurodegenerative disorders. However, the structural mechanisms by which intermediates initiate fibrillar aggregation have remained largely elusive. To gain insight, we used CPMG Relaxation dispersion NMR spectroscopy to determine the atomic-resolution three-dimensional solution structure of a 2% populated, on-pathway folding intermediate of the A39V/N53P/V55L Fyn SH3 domain. To this end, we used the backbone chemical shifts and RDCs/RCSAs of the “invisible” intermediate reconstructed from CPMG experiments as experimental input for structure calculations based on chemical shift restrained replica exchange molecular dynamics simulations via the CamShift approach (1). The COOH-terminus remains disordered in this intermediate (2), thereby exposing the aggregation-prone NH 2-terminal beta-strand. Accordingly, mutants lacking the COOH-terminus and thus mimicking the intermediatea fail to safeguard the folding route and spontaneously form β-sheet-rich fibrillar aggregates with a diameter of several nanometers and an affinity for the dye Congo red. The structure provides a detailed characterization of the non-native interactions stabilizing an aggregation-prone intermediate under native conditions and insight into how such an intermediate can derail folding and initiate fibrillation