Genetically directed synthesis and spectroscopic analysis of a protein polymer derived from a flagelliform silk sequence

Structural Diversity of Native Major Ampullate, Minor Ampullate, Cylindriform, and Flagelliform Silk Proteins in Solution

The foundations of silk spinning, the structure, storage, and activation of silk proteins, remain highly debated. By combining solution small-angle neutron and X-ray scattering (SANS and SAXS) alongside circular dichroism (CD), we reveal a shape anisotropy of the four principal native spider silk feedstocks from Nephila edulis. We show that these proteins behave in solution like elongated semiflexible polymers with locally rigid sections. We demonstrated that minor ampullate and cylindriform proteins adopt a monomeric conformation, while major ampullate and flagelliform proteins have a preference for dimerization. From an evolutionary perspective, we propose that such dimerization arose to help the processing of disordered silk proteins. Collectively, our results provide insights into the molecular-scale processing of silk, uncovering a degree of evolutionary convergence in protein structures and chemistry that supports the macroscale micellar/pseudo liquid crystalline spinning mechanisms proposed by the community.

Microbial production of amino acid-modified spider dragline silk protein with intensively improved mechanical properties

Spider dragline silk is a remarkably strong fiber with impressive mechanical properties, which were thought to result from the specific structures of the underlying proteins and their molecular size. In this study, silk protein 11R26 from the dragline silk protein of Nephila clavipes was used to analyze the potential effects of the special amino acids on the function of 11R26. Three protein derivatives, ZF4, ZF5, and ZF6, were obtained by site-directed mutagenesis, based on the sequence of 11R26, and among these derivatives, serine was replaced with cysteine, isoleucine, and arginine, respectively. After these were expressed and purified, the mechanical performance of the fibers derived from the four proteins was tested. Both hardness and average elastic modulus of ZF4 fiber increased 2.2 times compared with those of 11R26. The number of disulfide bonds in ZF4 protein was 4.67 times that of 11R26, which implied that disulfide bonds outside the poly-Ala region affect the mechanical properties of spider silk more efficiently. The results indicated that the mechanical performances of spider silk proteins with small molecular size can be enhanced by modification of the amino acids residues. Our research not only has shown the feasibility of large-scale production of spider silk proteins but also provides valuable information for protein rational design.

The correlation between the length of repetitive domain and mechanical properties of the recombinant flagelliform spidroin

Spider silk is an attractive biopolymer with numerous potential applications due to its remarkable characteristics. Among the six categories of spider silks, flagelliform (Flag) spider silk possesses longer and more repetitive core domains than others, therefore performing the highest extensibility. To investigate the correlation between the recombinant spidroin size and the synthetic fiber properties, four recombinant proteins with different sizes [N-Scn-C (n=1-4)] were constructed and expressed using IMPACT system. Subsequently, different recombinant spidroins were spun into fibers through wet-spinning via a custom-made continuous post-drawing device. Mechanical tests of the synthetic fibers with four parameters (maximum stress, maximum extension, Young's modulus and toughness) demonstrated that the extensibility of the fibers showed a positive correlation with spidroin size, consequently resulting in the extensibility of N-Sc4-C fiber ranked the highest (58.76%) among four fibers. Raman data revealed the relationship between secondary structure content and mechanical properties. The data here provide a deeper insight into the relationship between the function and structure of Flag silk for future design of artificial fibers.

Summary: A study of the relationship between the structure and property of synthetic spider silk-like fibers, which aims to aid with the designing of functional artificial fibers.

Fibrous proteins: At the crossroads of genetic engineering and biotechnological applications

Fibrous proteins, such as silk, elastin and collagen are finding broad impact in biomaterial systems for a range of biomedical and industrial applications. Some of the key advantages of biosynthetic fibrous proteins compared to synthetic polymers include the tailorability of sequence, protein size, degradation pattern, and mechanical properties. Recombinant DNA production and precise control over genetic sequence of these proteins allows expansion and fine tuning of material properties to meet the needs for specific applications. We review current approaches in the design, cloning, and expression of fibrous proteins, with a focus on strategies utilized to meet the challenges of repetitive fibrous protein production. We discuss recent advances in understanding the fundamental basis of structure-function relationships and the designs that foster fibrous protein self-assembly towards predictable architectures and properties for a range of applications. We highlight the potential of functionalization through genetic engineering to design fibrous protein systems for biotechnological and biomedical applications.

Biomimetic repeat protein derived from Xenopus tropicalis for fibrous scaffold fabrication

Collagen, silk, and elastin are the fibrous proteins consist of representative amino acid repeats. Because these proteins exhibited distinguishing mechanical properties, they have been utilized in diverse applications, such as fiber-based sensors, filtration membranes, supporting materials, and tissue engineering scaffolds. Despite their infinite prevalence and potential, most studies have only focused on a few repeat proteins. In this work, the hypothetical protein with a repeat motif derived from the frog Xenopus tropicalis was obtained and characterized for its potential as a novel protein-based material. The codon-optimized recombinant frog repeat protein, referred to as 'xetro', was produced at a high rate in a bacterial system, and an acid extraction-based purified xetro protein was successfully fabricated into microfibers and nanofibers using wet spinning and electrospinning, respectively. Specifically, the wet-spun xetro microfibers demonstrated about 2- and 1.5-fold higher tensile strength compared with synthetic polymer polylactic acid and cross-linked collagen, respectively. In addition, the wet-spun xetro microfibers showed about sevenfold greater stiffness than collagen. Therefore, the mass production potential and greater mechanical properties of the xetro fiber may result in these fibers becoming a new promising fiber-based material for biomedical engineering.

Rapid communication: Computational simulation and analysis of a candidate for the design of a novel silk-based biopolymer

This work theoretically investigates the mechanical properties of a novel silk-derived biopolymer as polymerized in silico from sericin and elastin-like monomers. Molecular Dynamics simulations and Steered Molecular Dynamics were the principal computational methods used, the latter of which applies an external force onto the system and thereby enables an observation of its response to stress. The models explored herein are single-molecule approximations, and primarily serve as tools in a rational design process for the preliminary assessment of properties in a new material candidate.

Facile and rapid ruthenium mediated photo-crosslinking of Bombyx mori silk fibroin

We report a unique and facile way of preparing silk fibroin gel by ruthenium-mediated photocrosslinking of silk solution. Compared to existing methods, this approach is faster, taking only a few minutes to form the gel with tunable modulus. Hydrogels demonstrate their potential suitability as biomaterials for tissue engineering applications.

Nephila clavipes Flagelliform silk-like GGX motifs contribute to extensibility and spacer motifs contribute to strength in synthetic spider silk fibers

Flagelliform spider silk is the most extensible silk fiber produced by orb weaver spiders, though not as strong as the dragline silk of the spider. The motifs found in the core of the Nephila clavipes flagelliform Flag protein are GGX, spacer, and GPGGX. Flag does not contain the polyalanine motif known to provide the strength of dragline silk. To investigate the source of flagelliform fiber strength, four recombinant proteins were produced containing variations of the three core motifs of the Nephila clavipes flagelliform Flag protein that produces this type of fiber. The as-spun fibers were processed in 80% aqueous isopropanol using a standardized process for all four fiber types, which produced improved mechanical properties. Mechanical testing of the recombinant proteins determined that the GGX motif contributes extensibility and the spacer motif contributes strength to the recombinant fibers. Recombinant protein fibers containing the spacer motif were stronger than the proteins constructed without the spacer that contained only the GGX motif or the combination of the GGX and GPGGX motifs. The mechanical and structural X-ray diffraction analysis of the recombinant fibers provide data that suggests a functional role of the spacer motif that produces tensile strength, though the spacer motif is not clearly defined structurally. These results indicate that the spacer is likely a primary contributor of strength, with the GGX motif supplying mobility to the protein network of native N. clavipes flagelliform silk fibers.

Recombinant production of spider silk proteins

Natural spider silk fibers combine extraordinary properties such as stability and flexibility which results in a toughness superseding that of all other fiber materials. As the spider's aggressive territorial behavior renders their farming not feasible, the biotechnological production of spider silk proteins (spidroins) is essential in order to investigate and employ them for applications. In order to accomplish this task, two approaches have been tested: firstly, the expression of partial cDNAs, and secondly, the expression of synthetic genes in several host organisms, including bacteria, yeast, plants, insect cells, mammalian cells, and transgenic animals. The experienced problems include genetic instability, limitations of the translational and transcriptional machinery, and low solubility of the produced proteins. Here, an overview of attempts to recombinantly produce spidroins will be given, and advantages and disadvantages of the different approaches and host organisms will be discussed.

Natural and Genetically Engineered Proteins for Tissue Engineering

To overcome the limitations of traditionally used autografts, allografts and, to a lesser extent, synthetic materials, there is the need to develop a new generation of scaffolds with adequate mechanical and structural support, control of cell attachment, migration, proliferation and differentiation and with bio-resorbable features. This suite of properties would allow the body to heal itself at the same rate as implant degradation. Genetic engineering offers a route to this level of control of biomaterial systems. The possibility of expressing biological components in nature and to modify or bioengineer them further, offers a path towards multifunctional biomaterial systems. This includes opportunities to generate new protein sequences, new self-assembling peptides or fusions of different bioactive domains or protein motifs. New protein sequences with tunable properties can be generated that can be used as new biomaterials. In this review we address some of the most frequently used proteins for tissue engineering and biomedical applications and describe the techniques most commonly used to functionalize protein-based biomaterials by combining them with bioactive molecules to enhance biological performance. We also highlight the use of genetic engineering, for protein heterologous expression and the synthesis of new protein-based biopolymers, focusing the advantages of these functionalized biopolymers when compared with their counterparts extracted directly from nature and modified by techniques such as physical adsorption or chemical modification.

Spider silk proteins: recent advances in recombinant production, structure-function relationships and biomedical applications

Spider dragline silk is an outstanding material made up of unique proteins-spidroins. Analysis of the amino acid sequences of full-length spidroins reveals a tripartite composition: an N-terminal non-repetitive domain, a highly repetitive central part composed of approximately 100 polyalanine/glycine rich co-segments and a C-terminal non-repetitive domain. Recent molecular data on the terminal domains suggest that these have different functions. The composite nature of spidroins allows for recombinant production of individual and combined regions. Miniaturized spidroins designed by linking the terminal domains with a limited number of repetitive segments recapitulate the properties of native spidroins to a surprisingly large extent, provided that they are produced and isolated in a manner that retains water solubility until fibre formation is triggered. Biocompatibility studies in cell culture or in vivo of native and recombinant spider silk indicate that they are surprisingly well tolerated, suggesting that recombinant spider silk has potential for biomedical applications.

Spider silk: understanding the structure-function relationship of a natural fiber

Spider silk is of great interest because of its extraordinary physical properties, such as strength and toughness. Here we discuss how these physical properties relate to the way in which spiders have utilized this material in prey capture, forcing its evolution to a high-performance fiber. Female spiders can produce up to seven different types of silk, and all these have different physical properties, which relate to their various functions. The variation in properties are due to underlying differences in the proteins making up these silks. As our understanding of spider silk has increased in the recent years, it has been possible to produce recombinant versions of the respective proteins. Recombinant proteins open up the potential to produce synthetic silk fibers with properties similar to those of the natural spider silk threads.

Multivalent protein polymers with controlled chemical and physical properties

In this review, we describe our work on the design, characterization, and modification of a series of alanine-rich helical polypeptides with novel functions. Glycosylation of the polypeptides has permitted investigation of polymer architecture effects on multivalent interactions. One of the members of this polypeptide family exhibits polymorphological behavior that is easily manipulated via simple changes in solution pH and temperature. Polypeptide-based fibrils formed at acidic pH and high temperature were shown to direct the one-dimensional organization of gold nanoparticles via electrostatic interactions. As a precursor to fibrils, aggregates likely comprising alanine-rich cores form at low temperatures and acidic pH and reversibly dissociate into monomers upon deprotonation. PEGylation of these polypeptides does not alter the self-association or conformational behavior of the polypeptide, suggesting potential applications in the development of assembled delivery vehicles, as modification of the polypeptides should be a useful strategy for controlling assembly.

Rational design of responsive self-assembling peptides from native protein sequences

This study used identified functional native domains from spider flagelliform silk protein and the Ca(2+) binding domain of lipase Lip A from Serratia marcescens . After carefully comparing the primary structures of both sequences, we rationally designed a newly sequenced eD(2) by "hiding" the ion binding sequence in the silk structure sequence. This helped avoid redundancy, and the new sequence had properties of both model sequences. In water, eD(2) formed uniform spherical agglomerates with a β-spiral structure. Triggered by Ca(2+), eD(2) formed nanofibers with higher compliance and thermal stability. We demonstrated the specialties of this novel peptide design by changing the pH, using other metal ions, and mutating the model sequence.

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.

Recovery in viscid line fibers

The development of a reliable procedure for removing the viscous coating of viscid silk has allowed the accurate characterization of the tensile behavior of clean flagelliform silk (i.e., silk of the flagelliform gland without the viscous coating synthetised in the aggregate gland). For comparison, tensile tests on native viscid silk (with the viscous coating) fibers were also performed. It was found that viscid silk, either native or clean, has an elastomeric behavior when kept wet, either by immersion in water (clean fibers) or by the effect of the viscid coating (native fibers). When tested in dry environments (35% RH, relative humidity, for clean fibers and 10% RH for native fibers), their mechanical behavior was no longer elastomeric, with it being more similar to other silk fibers. Furthermore, it was noticed that flagelliform silk fibers show a ground state to which they can return independent of the previous loading history.

A protocol for the production of recombinant spider silk-like proteins for artificial fiber spinning

The extreme strength and elasticity of spider silks originate from the modular nature of their repetitive proteins. To exploit such materials and mimic spider silks, comprehensive strategies to produce and spin recombinant fibrous proteins are necessary. This protocol describes silk gene design and cloning, protein expression in bacteria, recombinant protein purification and fiber formation. With an improved gene construction and cloning scheme, this technique is adaptable for the production of any repetitive fibrous proteins, and ensures the exact reproduction of native repeat sequences, analogs or chimeric versions. The proteins are solubilized in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at 25-30% (wt/vol) for extrusion into fibers. This protocol, routinely used to spin single micrometer-size fibers from several recombinant silk-like proteins from different spider species, is a powerful tool to generate protein libraries with corresponding fibers for structure-function relationship investigations in protein-based biomaterials. This protocol may be completed in 40 d.

Molecular design of performance proteins with repetitive sequences: recombinant flagelliform spider silk as basis for biomaterials

Most performance proteins responsible for the mechanical stability of cells and organisms reveal highly repetitive sequences. Mimicking such performance proteins is of high interest for the design of nanostructured biomaterials. In this article, flagelliform silk is exemplary introduced to describe a general principle for designing genes of repetitive performance proteins for recombinant expression in Escherichia coli . In the first step, repeating amino acid sequence motifs are reversely transcripted into DNA cassettes, which can in a second step be seamlessly ligated, yielding a designed gene. Recombinant expression thereof leads to proteins mimicking the natural ones. The recombinant proteins can be assembled into nanostructured materials in a controlled manner, allowing their use in several applications.

Understanding marine mussel adhesion

In addition to identifying the proteins that have a role in underwater adhesion by marine mussels, research efforts have focused on identifying the genes responsible for the adhesive proteins, environmental factors that may influence protein production, and strategies for producing natural adhesives similar to the native mussel adhesive proteins. The production-scale availability of recombinant mussel adhesive proteins will enable researchers to formulate adhesives that are water-impervious and ecologically safe and can bind materials ranging from glass, plastics, metals, and wood to materials, such as bone or teeth, biological organisms, and other chemicals or molecules. Unfortunately, as of yet scientists have been unable to duplicate the processes that marine mussels use to create adhesive structures. This study provides a background on adhesive proteins identified in the blue mussel, Mytilus edulis, and introduces our research interests and discusses the future for continued research related to mussel adhesion.

Biotechnological Production of Spider-Silk Proteins Enables New Applications

The outstanding mechanical properties of spider silks have motivated many researchers to establish biotechnological production techniques which are necessary to provide sufficient amounts of silk proteins for industrial applications. Based on recent developments in genetic engineering, two strategies for the recombinant production of spider-silk proteins have been established which are discussed in detail. Further, protein-design strategies are described, enabling the combination of silk properties with additional biological, chemical, or technical features. We highlight the potential of engineered and recombinantly-produced spider-silk proteins to provide the basis for a new generation of biomaterials.

Design and preparation of beta-sheet forming repetitive and block-copolymerized polypeptides

The design and rapid construction of libraries of genes coding beta-sheet forming repetitive and block-copolymerized polypeptides bearing various C- and N-terminal sequences are described. The design was based on the assembly of DNA cassettes coding for the (GA)3GX amino acid sequence where the (GAGAGA) sequences would constitute the beta-strand units of a larger beta-sheet assembly. The edges of this beta-sheet would be functionalized by the turn-inducing amino acids (GX). The polypeptides were expressed in Escherichia coli using conventional vectors and were purified by Ni-nitriloacetic acid (NTA) chromatography. The correlation of polymer structure with molecular weight was investigated by gel electrophoresis and mass spectrometry. The monomer sequences and post-translational chemical modifications were found to influence the mobility of the polypeptides over the full range of polypeptide molecular weights while the electrophoretic mobility of lower molecular weight polypeptides was more susceptible to C- and N-termini polypeptide modifications.

Thermally Associating Polypeptides Designed for Drug Delivery Produced by Genetically Engineered Cells

Thermally associating polymers, including gelatin, cellulose ethers (e.g., Methocels and poloxamers (e.g., Pluronics) have a long history of use in pharmacy. Over the past 20 years, significant advances in genetic engineering and the understanding of protein secondary and tertiary structures have been made. This has led to the development of a variety of polypeptides that do not occur naturally but can be expressed in recombinant cells and have useful properties that lend themselves to novel applications where current materials cannot perform. The most intensively studied motifs are derived from the consensus repeats of elastin and silk, as well as coiled-coil helices. Many of these designed polypeptides or 'artificial proteins' are thermally associating materials. This property can be exploited to develop solid dosage forms, injectable drug delivery systems, micro- or nanoparticle drug carriers, triggered or targeted release systems, or as a means of simplifying the purification process and thus reducing costs of production of these materials. This review focuses on the development and characterization of this novel class of biomaterials and examines their potential for pharmaceutical applications.

Unraveling the mechanical properties of composite silk threads spun by cribellate orb-weaving spiders

Orb-web weaving spiders depend upon the mechanical performance of capture threads to absorb the energy of flying prey. Most orb-weavers spin wet capture threads with core fibers of flagelliform silk. These threads are extremely compliant and extensible due to the folding of their constituent proteins into molecular nanosprings and hydration by a surrounding coating of aqueous glue. In contrast, other orb-weavers use cribellate capture threads, which are composite structures consisting of core fibers of pseudoflagelliform silk surrounded by a matrix of fine dry cribellar fibrils. Based on phylogenetic evidence, cribellate capture threads predate the use of viscid capture threads. To better characterize how pseudoflagelliform and cribellar fibrils function, we investigated the mechanical performance of cribellate capture threads for three genera of spiders (Deinopis, Hyptiotes and Uloborus). These taxa spin very diverse web architectures, ranging from complete orbs to evolutionarily reduced triangle webs and cast nets. We found that the pseudoflagelliform core fibers of these webs were stiffer and stronger, but also less extensible, than flagelliform silk. However,cribellate capture threads achieved overall high extensibilities because the surrounding cribellar fibrils contributed substantially to the tensile performance of threads long after the core pseudoflagelliform fibers ruptured. In the case of Deinopis capture threads, up to 90% of the total work performed could be attributed to these fibrils. These findings yield insight into the evolutionary transition from cribellate to viscid capture threads.

Bilayer fibril formation by genetically engineered polypeptides: preparation and characterization

A de novo, genetically engineered 687 residue polypeptide expressed in E. coli has been found to form highly rectilinear, beta-sheet containing fibrillar structures. Tapping-mode atomic force microscopy, deep-UV Raman spectroscopy, and transmission electron microscopy definitively established the tendency of the fibrils to predominantly display an apparently planar bilayer or ribbon assemblage. The ordered self-assembly of designed, extremely repetitive, high molecular weight peptides is a harbinger of the utility of similar materials in nanoscience and engineering applications.

Solid-State NMR Analysis of a Peptide (Gly-Pro-Gly-Gly-Ala)6-Gly Derived from a Flagelliform Silk Sequence of Nephila clavipes

Solid-state NMR is especially useful when the structures of peptides and proteins should be analyzed by taking into account the structural distribution, that is, the distribution of the torsion angle of the individual residue. In this study, two-dimensional spin-diffusion solid-state NMR spectra of 13C-double-labeled model peptides (GPGGA)6G of flagelliform silk were observed for studying the local structure in the solid state. The spin-diffusion NMR spectra calculated by assuming the torsion angles of the beta-spiral structure exclusively could not reproduce the observed spectra. In contrast, the spectra calculated by taking into account the statistical distribution of the torsion angles of the individual central residues in the sequences Ala-Gly-Pro, Gly-Pro-Gly, Pro-Gly-Gly, Gly-Gly-Ala, and Gly-Ala-Gly from PDB data could reproduce the observed spectra well. This indicates that the statistical distribution of the torsion angles should be considered for the structural model of (GPGGA)6G similar to the case of the model peptide of elastin.

Biopolymer-based biomaterials as scaffolds for tissue engineering

Biopolymers as biomaterials and matrices in tissue engineering offer important options in control of structure, morphology and chemistry as reasonable substitutes or mimics of extracellular matrix systems. These features also provide for control of material functions such as mechanical properties in gel, fiber and porous scaffold formats. The inherent biodegradability of biopolymers is important to help regulate the rate and extent of cell and tissue remodeling in vitro or in vivo. The ability to genetically redesign these polymer systems to bioengineer appropriate features to regulate cell responses and interactions is another important feature that offers both fundamental insight into chemistry-structure-function relationships as well as direct utility as biomaterials. Biopolymer matrices for biomaterials and tissue engineering can directly influence the functional attributes of tissues formed on these materials and suggest they will continue play an increasingly important role in the field.

Synthesis of and Structural Studies on Repeating Sequences of Abductin

Little data exist on the structure and function of compressible elastomeric proteins such as abductin. An understanding of the underlying structural features of these proteins may lead to the development of a new class of highly tailored "compressible" hydrogels. To that effect, in this work, the structure of abductin was investigated by means of studies on several synthetic peptides corresponding to the most frequent sequences of abductin. In particular, the 10 amino acid abductin peptide sequence FGGMGGGNAG, tandem repeated in the protein, and two related 25 and 40 amino acid polypeptides were synthesized. These peptides were studied with regard to secondary structure, self-assembly, and polymer morphology. The results obtained with these peptides allow us to propose a preliminary structure-elasticity relationship for abductin not dissimilar from that currently accepted for elastin.A possible mechanism of elasticity relating abductin to elastin.

Conformational behavior of chemically reactive alanine-rich repetitive protein polymers

The synthesis of protein-based polymers with controlled conformational properties and functional group placement offers many opportunities for the design of advanced materials. In this work, protein engineering methods have been used to produce repetitive alanine-rich protein polymers with the sequence [(AAAQ)(5)(AAAE)(AAAQ)(5)](x) (x = 2 and 6); these macromolecules may mimic architectural features of certain alanine-rich helical sequences found in natural proteins. Various proteins from this family can be readily expressed and purified from Escherichia coli. Circular dichroic spectroscopy (CD) characterization demonstrates that the purified proteins are highly helical under a variety of conditions. Thermal analysis of [(AAAQ)(5)(AAAE)(AAAQ)(5)](2) via differential scanning calorimetry (DSC) and CD indicates that the protein undergoes a reversible helix-coil transition at approximately 45 degrees C and that the protein conformation can be manipulated at elevated temperatures depending on solution conditions. The demonstrated conformational properties of these artificial proteins suggest that they may be excellent candidates for elucidating structure-function relationships in biopolymers for nanotechnology and biological applications.

Spider silks: recombinant synthesis, assembly, spinning, and engineering of synthetic proteins

Since thousands of years humans have utilized insect silks for their own benefit and comfort. The most famous example is the use of reeled silkworm silk from Bombyx mori to produce textiles. In contrast, despite the more promising properties of their silk, spiders have not been domesticated for large-scale or even industrial applications, since farming the spiders is not commercially viable due to their highly territorial and cannibalistic nature. Before spider silks can be copied or mimicked, not only the sequence of the underlying proteins but also their functions have to be resolved. Several attempts to recombinantly produce spider silks or spider silk mimics in various expression hosts have been reported previously. A new protein engineering approach, which combines synthetic repetitive silk sequences with authentic silk domains, reveals proteins that closely resemble silk proteins and that can be produced at high yields, which provides a basis for cost-efficient large scale production of spider silk-like proteins.

Secondary Structures and Conformational Changes in Flagelliform, Cylindrical, Major, and Minor Ampullate Silk Proteins. Temperature and Concentration Effects

Orb weaver spiders use exceptionally complex spinning processes to transform soluble silk proteins into solid fibers with specific functions and mechanical properties. In this study, to understand the nature of this transformation we investigated the structural changes of the soluble silk proteins from the major ampullate gland (web radial threads and spider safety line); flagelliform gland (web sticky spiral threads); minor ampullate gland (web auxiliary spiral threads); and cylindrical gland (egg sac silk). Using circular dichroism, we elucidated (i) the different structures and folds for the various silk proteins; (ii) irreversible temperature-induced transitions of the various silk structures toward beta-sheet-rich final states; and (iii) the role of protein concentration in silk storage and transport. We discuss the implication of these results in the spinning process and a possible mechanism for temperature-induced beta-sheet formation.

Characterization of Glutamine Deamidation in a Long, Repetitive Protein Polymer via Bioconjugate Capillary Electrophoresis

We describe a novel method for the determination of glutamine deamidation in a long protein polymer via bioconjugate capillary electrophoresis. Since the current best technique for detection of glutamine (or asparagine) deamidation is mass spectrometry, it is practically impossible to precisely detect the degree of deamidation (i.e., how many residues are deamidated in a polypeptide) in a large protein containing a significant number of glutamine (or asparagine) residues, because the mass difference between native and deamidated residues is just 1 atomic mass unit. However, by covalently attaching polydisperse protein polymers (337 residues) to a monodisperse DNA oligomer (22 bases), the degree of glutamine deamidation, which could not be determined accurately by mass spectrometry, was resolved by free-solution capillary electrophoresis. Electrophoretic separations were carried out after different durations of exposure of the protein to a cyanogen bromide cleavage reaction mixture, which is a general treatment for the purpose of removing an oligopeptide affinity purification tag from fusion proteins. For protein polymers with increasing extents of deamidation, the electromotive force of DNA + polypeptide conjugate molecules increases due to the introduced negative charge of deamidated glutamic acid residues, and consequently CE analysis reveals increasing differences in the electrophoretic mobilities of conjugate molecules, which qualitatively shows the degree of deamidation. Peak analysis of the electropherograms enables quantitative determination of the first four deamidations in a protein polymer. A first-order rate constant of 0.018 h(-1) was determined for the deamidation of a single glutamine residue in the protein polymer during the cyanogen bromide cleavage reaction.

Cloning, expression, and assembly of sericin-like protein

Recombinant sericin proteins of different molecular masses (17.4, 31.9, and 46.5 kDa), based on the 38-amino acid repetitive motif of native sericin, were cloned, expressed, and purified. The recombinant sericin self-assembled during dialysis (starting concentration of 2.5 mg/ml) forming twisted fibers. Circular dichroism and Fourier transform infrared spectroscopy studies demonstrated protein conformational transitions occurred from random coil to beta-sheets during the dialysis. Congo red-stained recombinant sericin fibrils exhibited apple-green birefringence, indicating long-range order in the array of beta-sheets. Biosynthetic sericin has a high content of polar amino acids (e.g. > 40 mol % serine), leading to a beta-sheet conformation formed by hydrogen bonding via polar zipper interactions. Analysis of recombinant sericin sequence using Mandel-Gutfreund's (Mandel-Gutfreund, Y., and Gregoret, L. M. (2002) J. Mol. Biol. 323, 453-461) definition of polar and non-polar amino acids showed that the hydrophobicity pattern resembles the most frequent pattern of amyloidogenic proteins, polar amino acid aggregates (PPPPP). Many beta-proteins and peptides are designed to study amyloidogenesis using a polar/non-polar alternating pattern (PNPNPN). Sericin-like proteins or peptides provide an alternative model in terms of hydrophobicity pattern with which to explore questions related to beta-sheet formation and amyloidogenesis. The glue-like property of sericin is attributed to the hydrogen bonding between serine residues of sericin with serine residues in the fibroin structural components of silk fiber.

Design of nanostructured biological materials through self-assembly of peptides and proteins

Several self-assembling peptide and protein systems that form nanotubes, helical ribbons and fibrous scaffolds have recently emerged as biological materials. Peptides and proteins have also been selected to bind metals, semiconductors and ions, inspiring the design of new materials for a wide range of applications in nano-biotechnology.

Genetic engineering of fibrous proteins: spider dragline silk and collagen

Various strategies have been employed to genetically engineer fibrous proteins. Two examples, the subject of this review, include spider dragline silk from Nephila clavipes and collagen. These proteins are highlighted because of their unique mechanical and biological properties related to controlled release, biomaterials and tissue engineering. Cloning and expression of native genes and synthetic artificial variants of the consensus sequence repeats from the native genes has been accomplished. Expression of recombinant silk and collagen proteins has been reported in a variety of host systems, including bacteria, yeast, insect cells, plants and mammalian cells. Future utility for these proteins for biomedical materials is expected to increase as needs expand for designer materials with tailored mechanical properties and biological interactions to elicit specific responses in vitro and in vivo.