Spectroscopic studies of Manduca sexta and Sesamia nonagrioides chorion protein structure

Abstract

The secondary structure of Manduca sexta and Sesamia nonagrioides chorion proteins has been studied in intact chorions using laser-Raman and Fourier transform infra-red (FTIR) spectroscopy and in a solution containing extracted and reassembled chorion proteins using circular dichroism (CD) spectroscopy. Laser-Raman and IR spectra suggest the predominance of antiparallel β-pleated sheet structure in intact chorion proteins of both Lepidoptera species. The bands at 1673, 1674 cm −1 (amide I) and 1234–1238 cm −1 (amide III) in the laser-Raman spectra can best be interpreted as resulting from abundant antiparallel β-pleated sheet structure. Analysis of the amide I band suggests that chorion proteins consist of 60–70% antiparallel β-pleated sheet and 30–40% β-turns. Supporting evidence for the prevalence of antiparallel β-pleated sheet in chorion proteins was supplied using FTIR spectroscopy by the observation of a very intense absorption band at 1635 cm −1(amide I) and of a weak band at 1530, 1525 cm −1 (amide II) from chorions of both species. Surprisingly, analysis of the CD spectra of extracted and reassembled chorion proteins suggests that, in solution, they retain a regular secondary structure most probably dominated by β-pleated sheet. We therefore suggest that the prominent regular β-sheet structure of chorion proteins may exist in solution and dictate the aggregation and polymerization process in vivo.