Characterisation of the helicostatin peptide precursor gene from Helicoverpa armigera (Lepidoptera: Noctuidae)

The gene encoding the helicostatin peptide family was isolated from a Helicoverpa armigera genomic DNA library. The deduced precursor sequence allowed unambiguous identification of all helicostatin peptides and verified the sequences of eight peptides previously isolated. The gene consists of at least three exons and encodes a precursor of 225 amino acids that contains three blocks of tandemly arranged helicostatin peptides including seven copies of the C-terminal sequence -YXFGL followed by a single Gly residue for carboxylamidation. Complete endoproteolytic processing at all possible dibasic cleavage sites would generate the seven helicostatin octapeptides previously purified from larval extracts. If processing was not complete at the third pair of basic amino acids the octadecapeptide (helicostatin IIa) would also be released. Two novel putative helicostatin peptide sequences were identified; YSKFNFGL and ERDMHRFSFGL, both of which had the C-terminal pentapeptide -FXFGL in place of the more usual -YXFGL sequence. Comparison of the helicostatin precursor with that of the cockroaches, locust and flies revealed variation in size, sequence and organisation of the 'allatostatin' precursors across different insect orders. In situ hybridisation histochemistry established that helicostatins are expressed in neurones of the central nervous system and endocrine cells of the midgut, indicating that the helicostatins are true brain-gut peptides. Northern blot analysis identified a single transcript of 1.6 kb in mRNA from whole larvae, isolated central nervous system and gut tissue.