Regenerated Protein Fibers

Keywords

Honeybee silk • Coiled-coil protein • Transgenic expression • E. coli • Protein component • Fiber properties

Unlike Bombyx mori or spider silks that consist of large repetitive sequences and P-sheets, honeybees secrete four different types of small coiled-coil proteins in nearly equal proportions with a molecular weight of about 30 kDa [08Shi, 10Wei]. These proteins are non-repetitive and are rich in alanine residues, and the proteins were found to be stable in water [08Shi]. Due to their lower molecular weights and unique structure, it was supposed that honeybee silks could easily produce recombinant proteins. To examine this, four proteins (ABS 1-4 with 315, 289, 317, and 321 residues) from the Asiatic honeybee were expressed in Escherichia coli, and the structures of the proteins were studied. The yield of proteins in E. coli was 30, 30, 10, and 60 mg/mL for ABS-1-4, respectively. Corresponding molecular weights obtained for the proteins were 55, 32, 38, and 50 kDa, respectively. Proteins generated were found to have about 65 % coiled-coil sequences but with low (9-27 %) a-helix content and high% (45-56 %) of random coils. In addition, about 26-35 % of p-sheets were also discovered [08Shi]. Some of the properties of the recombinant honeybee silk proteins expressed in E. coli are listed in Table 51.1. Overall, it is seen that the recombinant production of honeybee silks was unable to generate the secondary and tertiary structure seen in native honeybee silk [10Wei]. In the native honeybee silks, the four isolated proteins are found as a complex but have weak interactions between them. In solution, a-sheets, P-sheets, and random coils coexist depending on the pH of the solution. The presence of high amounts of alanine that provides limited hydrophobic interactions was suggested to be the reason for the inability of the silks to maintain higher levels of a- or p-helices [08Shi]. In a similar study, recombinant proteins with yields between 0.5 and 2.5 g/L were obtained using honeybees (Apis mellifera), and the proteins were formed into fibers [10Wei]. Four of the distinct honeybee proteins

were expressed in E. coli (Rosetta 2 DE3), and proteins were collected. Proteins containing all four components were concentrated to get the required viscosity, and the solution was manually drawn into fibers between the prongs of tweezers. Fibers were coagulated in 90 % methanol and 10 % water bath and drawn to about 2x the length and air-dried. Tensile properties of the fibers obtained are listed in Table 51.2.

The as-drawn fibers were soluble in water but became insoluble after coagula­tion in methanol indicating the transition from coiled coil to p structure. Drawing the fibers resulted in an alignment of the proteins and a substantial improvement in tensile properties as seen in Table 51.3. After drawing (1x), fibers had similar elongation and modulus but lower strength than native silk fibers. It was suggested that protein in honeybee glands pre-assembles into aligned liquid crystals before spinning. However, this notion has been proven to be a non-requisite for spider silks as discussed in an earlier chapter in this part. Further studies were done to under­stand the role of the four components in honeybee silks on fiber properties and to investigate if a single protein component was able to replicate the fiber properties obtained using a combination of all the four components [11Sut]. Of the four components, AmelF3 was found to be most suitable for fiber formation since it remained in solution even at high concentrations required for fiber formation, whereas the other components precipitated. Fibers were extruded from the AmelF3 fraction and from the combined AmelF1-4 components and drawn in a methanol bath. Properties of the fibers obtained from AmelF3 and the combined proteins are shown in Table 51.3. As seen in the table, fibers produced from AmelF1-4 had higher strength and lower extensibility when drawn to the same extent. However, AmelF3 will be about 50-60 % stronger than AmelF1-4 if fibers of equivalent diameters are considered. As with other silk fibers, drawing was found to increase the p-sheet content leading to increased tensile strength. It was suggested that the recombinant protein fibers consisted of coiled-coil structures

that were linked by p-sheets. The extent of alignment of p-sheets could be con­trolled during fiber formation. Fibers with higher amounts of coiled-coil structure with low drawing resulted in fibers with moderate strength and toughness, whereas fibers with higher levels of p-sheets were found to have higher strength but low extensibility [11Sut]. In a further continuation of their research, fibers were pro­duced from AmelF3 and then heated at 190 °C for 1 h to covalently cross-link the proteins [13Poo]. Fibers (34 pm) obtained had strength of 1.4 g/den and elongation of 42 % and were stable in protein denaturants such as urea and guanidium. These fibers were knitted and woven into a tubular sheet shown in Fig. 51.1 [13Poo].

References

[08Shi] Shi, J., Lua, S., Du, N., Liu, X., Song, J.: Biomaterials 29, 2820 (2008)

[10Wei] Weisman, S., Haritos, V. S., Church, J. S., Huson, M. G., Mudie, S. T., Rodgers, A. J.W., Dumsday, G. J., Sutherland, T. D.: Biomaterials 31, 2695 (2010)

[11Sut] Sutherland, T. D., Church, J. S., Hu, X., Huson, M. G., Kaplan, D. L., Weisman, S.: PLoS One 6(2), 1 (2011)

[13Poo] Poole, J., Church, J. S., Woodhead, A. L., Huson, M. G., Sriskantha, A., Kyaratzis, I. L., Sutherland, T. D.: Macromol. Biosci. 13, 1321 (2013)