Structure and activity of the insect cytokine growth-blocking peptide

Growth-blocking peptide (GBP) is a 25-amino acid insect cytokine found in L epidopteran insects that possesses diverse biological activities such as la rval growth regulation, cell proliferation, and stimulation of immune cells (plasmatocytes). The tertiary structure of GBP consists of a structured co re that contains a disulfide bridge and a short antiparallel beta -sheet (T yr(11)Arg(13) and Cys(19)-Pro(21)) and flexible N and C termini (Glu(1)-Gly (6) and Phe(23)-Gln(25)). In this study, deletion and point mutation analog s of GBP were synthesized to investigate the relationship between the struc ture of GBP and its mitogenic and plasmatocyte spreading activity. The resu lts indicated that deletion of the N-terminal residue, Glu(1), eliminated a ll plasmatocyte spreading activity but did not reduce mitogenic activity. I n contrast, deletion of Phe(23) along with the remainder of the C terminus destroyed all mitogenic activity but only slightly reduced plasmatocyte spr eading activity. Therefore, the minimal structure of GBP containing mitogen ic activity is 2-23 GBP, whereas that with plasmatocyte spreading activity is 1-22 GBP. NMR analysis indicated that these N- and C-terminal deletion m utants retained a similar core structure to wild-type GBP. Replacement of A sp(16) with either a Glu, Leu, or Asn residue similarly did not alter the c ore structure of GBP. However, these mutants had no mitogenic activity, alt hough they retained about 50% of their plasmatocyte spreading activity. We conclude that specific residues in the unstructured and structured domains of GBP differentially affect the biological activities of GBP, which sugges ts the possibility that multifunctional properties of this peptide may be m ediated by different forms of a GBP receptor.