Synthesis of and Structural Studies on Repeating Sequences of Abductin

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Elastin-like polypeptides as models of intrinsically disordered proteins

Elastin-like polypeptides (ELPs) are a class of stimuli-responsive biopolymers inspired by the intrinsically disordered domains of tropoelastin that are composed of repeats of the VPGXG pentapeptide motif, where X is a "guest residue". They undergo a reversible, thermally triggered lower critical solution temperature (LCST) phase transition, which has been utilized for a variety of applications including protein purification, affinity capture, immunoassays, and drug delivery. ELPs have been extensively studied as protein polymers and as biomaterials, but their relationship to other disordered proteins has heretofore not been established. The biophysical properties of ELPs that lend them their unique material behavior are similar to the properties of many intrinsically disordered proteins (IDP). Their low sequence complexity, phase behavior, and elastic properties make them an interesting "minimal" artificial IDP, and the study of ELPs can hence provide insights into the behavior of other more complex IDPs. Motivated by this emerging realization of the similarities between ELPs and IDPs, this review discusses the biophysical properties of ELPs, their biomedical utility, and their relationship to other disordered polypeptide sequences.

Synthesis and characterization of recombinant abductin-based proteins

Recombinant proteins are promising tools for tissue engineering and drug delivery applications. Protein-based biomaterials have several advantages over natural and synthetic polymers, including precise control over amino acid composition and molecular weight, modular swapping of functional domains, and tunable mechanical and physical properties. In this work, we describe recombinant proteins based on abductin, an elastomeric protein that is found in the inner hinge of bivalves and functions as a coil spring to keep shells open. We illustrate, for the first time, the design, cloning, expression, and purification of a recombinant protein based on consensus abductin sequences derived from Argopecten irradians . The molecular weight of the protein was confirmed by mass spectrometry, and the protein was 94% pure. Circular dichroism studies showed that the dominant structures of abductin-based proteins were polyproline II helix structures in aqueous solution and type II β-turns in trifluoroethanol. Dynamic light scattering studies illustrated that the abductin-based proteins exhibit reversible upper critical solution temperature behavior and irreversible aggregation behavior at high temperatures. A LIVE/DEAD assay revealed that human umbilical vein endothelial cells had a viability of 98 ± 4% after being cultured for two days on the abductin-based protein. Initial cell spreading on the abductin-based protein was similar to that on bovine serum albumin. These studies thus demonstrate the potential of abductin-based proteins in tissue engineering and drug delivery applications due to the cytocompatibility and its response to temperature.

Structural disorder and protein elasticity

An emerging class of disordered proteins underlies the elasticity of many biological tissues. Elastomeric proteins are essential to the function of biological machinery as diverse as the human arterial wall, the capture spiral of spider webs and the jumping mechanism of fleas. In this chapter, we review what is known about the molecular basis and the functional role of structural disorder in protein elasticity. In general, the elastic recoil of proteins is due to a combination of internal energy and entropy. In rubber-like elastomeric proteins, the dominant driving force is the increased entropy of the relaxed state relative to the stretched state. Aggregates of these proteins are intrinsically disordered or fuzzy, with high polypeptide chain entropy. We focus our discussion on the sequence, structure and function of five rubber-like elastomeric proteins, elastin, resilin, spider silk, abductin and ColP. Although we group these disordered elastomers together into one class of proteins, they exhibit a broad range of sequence motifs, mechanical properties and biological functions. Understanding how sequence modulates both disorder and elasticity will help advance the rational design of elastic biomaterials such as artificial skin and vascular grafts.

Modelling the self-assembly of elastomeric proteins provides insights into the evolution of their domain architectures

Elastomeric proteins have evolved independently multiple times through evolution. Produced as monomers, they self-assemble into polymeric structures that impart properties of stretch and recoil. They are composed of an alternating domain architecture of elastomeric domains interspersed with cross-linking elements. While the former provide the elasticity as well as help drive the assembly process, the latter serve to stabilise the polymer. Changes in the number and arrangement of the elastomeric and cross-linking regions have been shown to significantly impact their assembly and mechanical properties. However, to date, such studies are relatively limited. Here we present a theoretical study that examines the impact of domain architecture on polymer assembly and integrity. At the core of this study is a novel simulation environment that uses a model of diffusion limited aggregation to simulate the self-assembly of rod-like particles with alternating domain architectures. Applying the model to different domain architectures, we generate a variety of aggregates which are subsequently analysed by graph-theoretic metrics to predict their structural integrity. Our results show that the relative length and number of elastomeric and cross-linking domains can significantly impact the morphology and structural integrity of the resultant polymeric structure. For example, the most highly connected polymers were those constructed from asymmetric rods consisting of relatively large cross-linking elements interspersed with smaller elastomeric domains. In addition to providing insights into the evolution of elastomeric proteins, simulations such as those presented here may prove valuable for the tuneable design of new molecules that may be exploited as useful biomaterials.

Elastomeric proteins such as elastin, resilin, abductin and wheat gluten represent a remarkable class of self-assembling proteins that provide properties of extensibility and elastic recoil. Although unrelated from an evolutionary viewpoint, these proteins nonetheless share a common sequence design involving highly repetitive elastomeric regions interspersed with elements capable of forming cross-links that help stabilize the formation of polymers. Attempts to explore the influence of domain architecture on the self-assembly and mechanical properties of elastomeric proteins at the molecular level have largely been hindered by a general lack of detailed structural information. Here we introduce a novel theoretical study based on random walks to simulate the self-assembly of elastomeric proteins. Applying this model, we explored the impact of different configurations of elastomeric and cross-linking elements on the stability of the resultant polymer. Through exploring the complex relationships between elastomeric domains, required to drive self-assembly, and cross-linking domains, required for structural integrity, results from these simulations provide insights into the molecular basis for the evolution of elastomeric proteins as well as help guide the rational design of novel elastomeric-peptides.

From bouncy legs to poisoned arrows: elastic movements in invertebrates

Elastic mechanisms in the invertebrates are fantastically diverse, yet much of this diversity can be captured by examining just a few fundamental physical principles. Our goals for this commentary are threefold. First, we aim to synthesize and simplify the fundamental principles underlying elastic mechanisms and show how different configurations of basic building blocks can be used for different functions. Second, we compare single rapid movements and rhythmic movements across six invertebrate examples – ranging from poisonous cnidarians to high-jumping froghoppers – and identify remarkable functional properties arising from their underlying elastic systems. Finally, we look to the future of this field and find two prime areas for exciting new discoveries – the evolutionary dynamics of elastic mechanisms and biomimicry of invertebrate elastic materials and mechanics.

Elastomeric polypeptide-based biomaterials

Elastomeric proteins are characterized by their large extensibility before rupture, reversible deformation without loss of energy, and high resilience upon stretching. Motivated by their unique mechanical properties, there has been tremendous research in understanding and manipulating elastomeric polypeptides, with most work conducted on the elastins but more recent work on an expanded set of polypeptide elastomers. Facilitated by biosynthetic strategies, it has been possible to manipulate the physical properties, conformation, and mechanical properties of these materials. Detailed understanding of the roles and organization of the natural structural proteins has permitted the design of elastomeric materials with engineered properties, and has thus expanded the scope of applications from elucidation of the mechanisms of elasticity to the development of advanced drug delivery systems and tissue engineering substrates.

Biofunctional design of elastin-like polymers for advanced applications in nanobiotechnology

Elastin-like recombinant protein polymers are a new family of polymers which are captivating the attention of a broad audience ranging from nanotechnologists to biomaterials and more basic scientists. This is due to the extraordinary confluence of different properties shown by this kind of material that are not found together in other polymer systems. Elastin-like polymers are extraordinarily biocompatible, acutely smart and show uncommon self-assembling capabilities. Additionally, they are highly versatile, since these properties can be tuned and expanded in many different ways by substituting the amino acids of the dominating repeating peptide or by inserting, in the polymer architecture, (bio)functional domains extracted from other natural proteins or de novo designs. Recently, the potential shown by elastin-like polymers has, in addition, been boosted and amplified by the use of recombinant DNA technologies. By this means, complex molecular designs and extreme control over the amino-acid sequence can be attained. Nowadays, the degree of complexity and control shown by the elastin-like protein polymers is well beyond the reach of even the most advanced polymer chemistry technologies. This will open new possibilities in obtaining synthetic advanced bio- and nanomaterials. This review explores the present development of elastin-like protein polymers, with a particular emphasis for biomedical uses, along with some future directions that this field will likely explore in the near future.

Proline and Glycine Control Protein Self-Organization into Elastomeric or Amyloid Fibrils

Elastin provides extensible tissues, including arteries and skin, with the propensity for elastic recoil, whereas amyloid fibrils are associated with tissue-degenerative diseases, such as Alzheimer's. Although both elastin-like and amyloid-like materials result from the self-organization of proteins into fibrils, the molecular basis of their differing physical properties is poorly understood. Using molecular simulations of monomeric and aggregated states, we demonstrate that elastin-like and amyloid-like peptides are separable on the basis of backbone hydration and peptide-peptide hydrogen bonding. The analysis of diverse sequences, including those of elastin, amyloids, spider silks, wheat gluten, and insect resilin, reveals a threshold in proline and glycine composition above which amyloid formation is impeded and elastomeric properties become apparent. The predictive capacity of this threshold is confirmed by the self-assembly of recombinant peptides into either amyloid or elastin-like fibrils. Our findings support a unified model of protein aggregation in which hydration and conformational disorder are fundamental requirements for elastomeric function.