Synthesis and characterization of chimeric silkworm silk.

A synthetic gene encoding a chimeric silklike protein was constructed that combined a polyalanine encoding region (Ala)18, a sequence slightly longer than the (Ala)12-13 found in the silk fibroin from the wild silkworm Samia cynthia ricini, and a sequence encoding GVGAGYGAGAGYGVGAGYGAGVGYGAGAGY, found in the silk fibroin from the silkworm Bombyx mori. A tetramer of the chimeric repeat sequence encoding a ∼29 kDa protein was expressed as a fusion protein in Escherichia coli. In comparison to S. c. ricini silk, the chimeric protein demonstrated improved solubility because it could be dissolved in 8 M urea. The purified protein assumed an α-helical structure based on solid-state 13C CP/MAS NMR and was less prone to conformational transition to a β-sheet, unlike native silk proteins from S. c. ricini. Model peptides representing the crystalline region of S. c. ricini silk fibroin, (Ala)12 and (Ala)18, formed β-sheet structures. Therefore, the solubility and structural transitions of the chimeric protein were ...