The core and complementary sequence responsible for biological activity of the diapause hormone of the silkworm, Bombyx mori

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To evaluate the structure-function relationship of the diapause hormone of the silkworm, Bombyx mori, the entire molecule and selected fragment and deleted analogues were chemically synthesized to compare their biological activity. The C- terminal pentapeptide amide was the shortest fragment that elicited 11% diapause eggs at maximum, indicating that this sequence is the core-active structure required for a biological response. The full biological response of about 70% diapause eggs was expressed by the C- terminal hexapeptide amide. However, an ED 50 value of this peptide amide was 1000-fold higher than that of the parent molecule. The serial elongation of peptide chain lengths toward the N- terminus brought about the sudden decrease in ED 50 values at two positions between Arg 9-Gly 10 and Thr 1-Asp 2. The deletion of duplicated sequence(s) located in the middle part of the molecule or the truncation of N- terminal region of the parent molecule increased ED 50 values but had no effects on response. Thus, N- terminal region and duplicated sequences act as the complementary structures for full potency of diapause hormone.