Secondary structure of synthetic peptides derived from the repeating unit of a giant secretory protein from Chironomus tentans.

The secretory proteins of Chironomus tentans larvae, which are used to construct underwater feeding and pupation tubes, assemble into complexes in vitro. Members of a family of 1000 kDa proteins, the spIs, appear to form the fibrous backbone of the assembled complexes. The spIs consist of a core of tandemly repeating units of 60 to 90 amino acids that can be subdivided into two regions: the subrepeat region, made up of short internal repeats, and the constant region, which lacks simple subrepeats. We have synthesized peptides representative of the constant and subrepeat regions of one of the spIs, and have examined their secondary structure using Fourier transform IR and CD spectroscopy. The IR spectrum of the constant peptide indicates that this peptide has alpha-helical regions and beta-turns. The CD spectrum confirms this. The IR spectrum of the subrepeat peptide is similar to that of the poly(Gly)II helix, and also may indicate the presence of beta-turns. The CD spectrum is consistent with this helical structure. Extrapolation of these results to intact spIs is in agreement with secondary structure prediction and modeling studies. Our results indicate that the alpha-helices and poly(Gly)II-like helices are not arranged as coiled-coils, which are often found in fibrous proteins. We suggest that these structural elements may be in an unusual arrangement in the spIs, organized as alternating alpha-helices and poly(Gly)II or collagen-like helices, interspersed with beta-turns.