Growth-blocking peptide (GBP) is an insect growth factor consisting of 25 amino acid residues that retards the development of lepidopteran larvae at high concentration while it stimulates larval growth at low concentration. In this study, we determined the solution structure of GBP by two-dimensional 1 H NMR spectroscopy. The structure contains a short segment of doublestranded -sheet involving residues 11-13 and 19 -21 and a type-II -turn in the loop region (residues 8 -11), whereas the N and C termini are disordered. This is the first report of the three-dimensional structure of the peptiderigic insect growth factor, and the structure of the well defined region of GBP was found to share similarity with that of the C-terminal domain of the epidermal growth factor (EGF). Because GBP has been reported to stimulate DNA synthesis of not only insect cells but also human keratinocyte cells at the same level with EGF, the structural similarity between GBP and EGF may lead to the interaction of GBP to EGF receptor.Endoparasitic wasps can alter the development of their hosts from larvae to pupae. This developmental disturbance allows the wasps to complete their larval growth and to emerge while the host is still in the larval stage, otherwise the wasp larvae would be trapped in the sclerotized pupal cuticles. Growthblocking peptide (GBP) 1 was originally isolated from the larval hemolymph of the host armyworm Pseudaletia separata whose development is halted in the last larval instar stage from parasitization by the parasitoid wasp Cotesia kariyai (1-3). The fact that injection of 20 pmol of GBP into early last instar larvae of the armyworm retards larval growth and causes more than a few days delay in pupation clearly shows its growth blocking activity (2, 3). We have since determined that GBP is a host gene product and that the change of titers of this peptide correlate well with the growth rate of insect larvae (4, 5). Recently, injection studies of GBP indicated that low concentrations (lower than 1 pmol/larva) stimulated larval growth, whereas high concentrations (higher than 10 pmol/larva) retarded growth (6). Furthermore, bioassay data revealed that several pmol/ml of GBP stimulated DNA synthesis of SF-9 insect cells and human keratinocyte cells, although several nmol/ml of GBP did not stimulate cell proliferation at all. Therefore, it is thought that GBP can act as a growth factor controlling the proliferation of a broad range of cells and the growth rate of whole larvae.To examine structural features of GBP, NMR was used in this study. The solution structure that we report here was obtained by two-dimensional 1 H NMR spectroscopy with the aid of distance geometry and simulated annealing. The threedimensional structure of GBP contains a short double-stranded -sheet and a -turn in the C-terminal region, while six residues in the N terminus and three residues in the C terminus are highly disordered.
EXPERIMENTAL PROCEDURES
1H NMR Spectroscopy-The 25-residue peptide GBP was prepared by the Peptide Institute (Osa...