Identification and functional study of fhx-L1, a major silk component in Bombyx mori

The silkworm cocoon was composed of fibroins, sericins, protease inhibitors, and proteins of unknown function. In this study, we focused on fhx-L1 (fibrohexamerin-like1), which was the homolog of fibroin fhx (fibrohexamerin). We identified 154 fhx family genes in 44 Lepidoptera insects, and seven fhx-Ls were found in Bombyx mori. Fhx-L1 was the most abundant of these proteins in silk and was specifically expressed in the silk gland. Immunofluorescence analysis showed that fhx-L1 was secreted into the whole sericin layers, similar to sericin1 (ser1). Western blotting revealed that the fhx-L1 protein contains N-linked oligosaccharide chains. CRISPR/Cas9-mediated gene editing was used to generate a homozygous mutant of fhx-L1 (fhx-L1^KO). The cocoon of fhx-L1^KO was larger and fluffier than that of the wild-type (WT), which was attributed to the lower adhesion between silk fibers. We also found that the content of β-sheet in the mutant silk was lower than in the WT silk, which resulted in further deterioration of the mechanical properties of the fhx-L1^KO silk. Our study revealed the properties and function of fhx-L1 as a major structural component in silk. Then, our study provided a potential insight for in-depth study of silk protein function.

Conformational changes and location of BSA upon immobilization on zeolitic imidazolate frameworks

The location and the conformational changes of proteins/enzymes immobilized within Metal Organic Frameworks (MOFs) are still poorly investigated and understood. Bovine serum albumin (BSA), used as a model protein, was immobilized within two different zeolitic imidazolate frameworks (ZIF-zni and ZIF-8). Pristine ZIFs and BSA@ZIFs were characterized by X-ray diffraction, small-angle X-ray scattering, scanning electron microscopy, confocal laser scanning microscopy, thermogravimetric analysis, micro-FTIR and confocal Raman spectroscopy to characterize MOFs structure and the protein location in the materials. Moreover, the secondary structure and conformation changes of BSA after immobilization on both ZIFs were studied with FTIR. BSA is located both in the inner and on the outer surface of MOFs, forming domains that span from the micro- to the nanoscale. BSA crystallinity (β-sheets + α-helices) increases up to 25 % and 40 % due to immobilization within ZIF-zni and ZIF-8, respectively, with a consequent reduction of β-turns.

Terahertz spectroscopy for interpreting the formation and hierarchical structures of silk fibroin oligopeptides

Determining the configuration and conformation of peptides is crucial for interpreting their structure-property relationships. In this work, we present nondestructive terahertz time-domain spectroscopy combined with density functional theory (DFT) and potential energy distribution (PED) analysis to identify the hierarchical structures of oligopeptides. The characteristic THz spectra of silk fibroin oligopeptides have been measured. Supported by DFT and PED analysis, the intrinsic differences among the dipeptides were identified by the collective vibrational modes of "R" groups and terminal groups linked by molecular chains of amido bonds or benzene rings. For tetrapeptides and hexapeptides, a few weak resonances and intensity differences were distinguished by the vibration mode of the molecular collective network formed by the interaction of amide planes and intramolecular hydrogen bond interactions. According to the THz absorption analyses of amide planes and intramolecular interactions within the molecular chains of silk fibroin oligopeptide isomer pairs, the formation and hierarchical structures were successfully interpreted using THz spectroscopy. This investigation develops a better understanding of the peptide formation mechanism, which further provides guidance in interpreting the formation of silk.

Ultrafine and High-Strength Silk Fibers Secreted by Bimolter Silkworms

Ultrafine fibers are widely employed because of their lightness, softness, and warmth retention. Although silkworm silk is one of the most applied natural silks, it is coarse and difficult to transform into ultrafine fibers. Thus, to obtain ultrafine high-performance silk fibers, we employed anti-juvenile hormones in this study to induce bimolter silkworms. We found that the bimolter cocoons were composed of densely packed thin fibers and small apertures, wherein the silk diameter was 54.9% less than that of trimolter silk. Further analysis revealed that the bimolter silk was cleaner and lighter than the control silk. In addition, it was stronger (739 MPa versus 497 MPa) and more stiffness (i.e., a higher Young's modulus) than the trimolter silk. FTIR and X-ray diffraction results revealed that the excellent mechanical properties of bimolter silk can be attributed to the higher β-sheet content and crystallinity. Chitin staining of the anterior silk gland suggested that the lumen is narrower in bimolters, which may lead to the formation of greater numbers of β-sheet structures in the silk. Therefore, this study reveals the relationship between the structures and mechanical properties of bimolter silk and provides a valuable reference for producing high-strength and ultrafine silk fibers.

Composition and in silico structural analysis of fibroin from liquid silk of non-mulberry silkworm Antheraea assamensis

Silk is spun from the liquid precursor known as liquid silk secreted from the posterior part and stored in the silk gland lumen with occurrence of many momentary events. The liquid silk in the silk gland is transformed to the spun silk fibre. In this study the elucidation of the protein components of liquid silk from the posterior part of the silk gland (PSG) of saturniid silkworm Antheraea assamensis along with its structural characterization has been reported. The 3D model of the N-terminal amorphous portion with some repeat crystalline motifs (19-255) of core protein fibroin has also been constructed. 1D and 2D electrophoresis revealed the homo-dimeric structure of the silk protein. Secondary structure analysis by Circular dichroism, FTIR spectroscopy showed α helical structural component as predominant conformation in the liquid silk. The crystalline structure investigated through X ray diffraction (XRD) analysis also revealed the presence of less ordered amorphous α helical conformation in the liquid silk. The 3D structural model proposed of the residues from 19 to 255 has revealed structural stability throughout the molecular dynamics simulation process. This study will provide the detailed structural information and in silico analysis of the core protein present in the liquid silk of PSG.

Understanding the structural degradation of South American historical silk: A Focal Plane Array (FPA) FTIR and multivariate analysis

Silk artifacts constitute an invaluable heritage, and to preserve such patrimony it is necessary to correlate the degradation of silk fibroin with the presence of dyes, pollutants, manufacturing techniques, etc. Fourier Transform Infrared Spectroscopy with a Focal plane array detector (FPA FTIR) provides structural information at the micron scale. We characterized the distribution of secondary structures in silk fibers for a large set of South American historical textiles, coupling FTIR with multivariate statistical analysis to correlate the protein structure with the age of the samples and the presence of dyes. We found that the pressure applied during attenuated total reflectance (ATR) measurements might induce structural changes in the fibers, producing similar spectra for pristine and aged samples. Reflectance spectra were thus used for the rigorous characterization of secondary structures. Some correlation was highlighted between the age of the samples (spanning over five centuries) and specific changes in their secondary structure. A correlation was found between the color of the samples and structural alterations, in agreement with the chemical nature of the dyes. Overall, we demonstrated the efficacy of reflectance FPA µ-FTIR, combined with multivariate analysis, for the rigorous and non-invasive description of protein secondary structures on large sets of samples.

Structural and Mechanical Properties of Silk from Different Instars of Bombyx mori

Silkworm silk has excellent mechanical properties, biocompatibility, and promising applications in the biomedical sector. Silkworms spin silk at the beginning and end of each of their five instar stages, as well as spinning mature silk after the fifth instar. We evaluated the mechanical properties and structure of 10 kinds of silk fibers from different stages. A tensile test showed that instar beginning silk, instar end silk, and mature silk possess distinct properties. Attenuated total reflectance Fourier-transform infrared spectroscopy and X-ray diffraction results showed that the excellent mechanical properties of instar end silk are attributed to higher β-sheet content and suitable crystallinity. Liquid chromatography-tandem mass spectrometry showed that P25 protein content in IV-E silk is 2.9× higher than that of cocoon silk. This study can offer guidelines for further biomimetic investigations into the design and manufacture of artificial silk protein fibers with novel function.