Homologues of fibroin L-chain and P25 of Bombyx mori are present in Dendrolimus spectabilis and Papilio xuthus but not detectable in Antheraea yamamai.

Low molecular mass protein components of fibroin, whose electrophoretic patterns before and after the reductive cleavage of disulfide bonds were similar to those of L-chain and P25 of Bombyx mori, were identified in fibroin samples of Bombyx mandarina, Dendrolimus spectabilis and Papilio xuthus but not of Antheraea yamamai. Fibroin of A. yamamai is suggested to form a dimer of H-chain. Full length cDNA sequences were cloned for the homologues of L-chain and P25 from B. mandarina, D. spectabilis and P. xuthus. The deduced sequences of L-chain and P25 of B. mandarina are almost identical to those of B. mori, each containing a single amino acid change. Homologues of L-chain and P25 of D. spectabilis and P. xuthus show about 50% overall identity, respectively, with those of B. mori, but essential structural features; i.e. the three Cys residues in an L-chain and the eight Cys residues and one of the potential N-glycosylation sites in P25, are conserved in both species. These results, together with the published results for Galleria mellonella, suggest that the three-components (H-chain, L-chain and P25) complex of fibroin is rather common among Lepidopteran silk-producing insects, in contrast to the H-H dimer type found in the saturnid silkworm.