TANDEMLY REPEATING PEPTIDE MOTIFS AND THEIR SECONDARY STRUCTURE IN CERATITIS-CAPITATA EGGSHELL PROTEINS CCS36 AND CCS38

Evidence from amino acid composition, Fourier transform analysis of primary structure and secondary structure prediction suggests a tripartite structure for Ceratitis capitata eggshell proteins Ccs36 and Ccs38, which consists of a central domain and two flanking 'arms'. The proteins, apparently, contain tandemly repeating peptide motifs specific for each domain of the tripartite structure. The central domain of both proteins, which exhibits extensive sequence homology with the corresponding domains of Drosophila melanogaster proteins s36 and s38, is formed by tandem repeats of an octapeptide -X-X-X-Z-Z-Z-Z-Z- (where X = large hydrophobic residue and Z = beta-turn former residue) and its variants. It is predicted to adopt a compact, most probably twisted, antiparallel beta-pleated sheet structure of beta-sheet strands regularly alternating with beta-turns or loops. The central domains of Ccs36 and Ccs38 share structural similarities, but they are recognizably different. The 'arms' of the proteins presumably serving for protein and species-specific functions differ substantially from those of Drosophila melanogaster. In Ccs36, the C-terminal 'arm' is formed by, almost precise, tandem repeats of an octapeptide -Y-X-A-A-P-A-A-S- (X = G or S), whereas the N-terminal 'arm' contains repeats of the octapeptide -Z-Z-Z-A-X-A-A-Z- (X = Q, N or E and Z a beta-turn former). In both 'arms' alpha-helices are predicted, alternating with beta-turns. In the Ccs38 C-terminal 'arm' nonapeptide repeats of the form -Y-Z-Z-Z-Z- (G, A)-Z-Q-Z- (Z = A, G, P or S) are observed, whereas the N-terminal includes repeats of an octapeptide motif -x-y-G-z-G-u-G-v- (x = I, S, y = G, Q). The latter are predicted as beta-sheet strands alternating with beta-turns, whereas, for the former the evidence is conflicting. The presence of these motifs suggests periodical patterns of dityrosine crosslinks, which harden the eggshell rendering it insoluble. Fourier transform infrared spectroscopy data from intact Ceratitis capitata eggshells support the validity of prediction.