Design, expression, and structural characterization of hybrid proteins of Samia cynthia ricini and Bombyx mori silk fibroins

Genetically engineering strategies have been applied to obtain the designed proteins [GGAGSGYGGGYGHGYGSDGG(GAGAGS)(3)](n) of varying chain lengths (n = 2, 4 or 6); SLP2, SLP4, and SLP6, respectively. The amino acid sequence of the monomeric unit represents the incorporation of the consensus repeat sequences of the glycine-rich region of Samia cynthia ricini followed by the repetitive crystalline region of Bombyx-mori silk fibroins. The oligonucleotides were designed to encode the desired monomeric unit of silk-like hybrid proteins, which were then polymerised and expressed in Escherichi coli system. The monomer, SLP1, was synthesized chemically using solid phase method. The recombinant proteins were identified by N-terminal amino-acid sequencing and Western blotting. The structural characterizations of SLP1 and SLP6 samples were performed using C-13 CP/MAS NMR in the solid state. The structure of SLP1 in the solid state was distorted beta-turn by judging from the chemical shift and line shape of the C-beta carbon of Ala residue, and the structural transition from distorted beta-turn to beta-sheet occurred partially in SLP6. The solution structure was studied in hexafluoro-2-propanol (HFIP) using CD method. Although both samples, SLP1 and SLP6, showed indication of the ordered structures, marked change in the band intensities was observed between two CD spectra. Especially, the CD pattern of SLP6 is in agreement with the pattern with right-handed 3(10)-helical conformation.