Demonstration of multiforms of insect cytokine receptors

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Growth-blocking peptide (GBP) is a member of an insect cytokine family. GBP is a small peptide consists of 25 amino acid residues, but it exerts diverse functions including larval growth retardation, cell proliferation, paralysis induction, cardioacceleration, immune cell stimulation, and morphogenesis. In this study, deletion analogs of GBP were synthesized to investigate the relationship between the structure of GBP and the biological activities such as mitogenic and plasmatocyte (a type of hemocytes) spreading activities. The results indicated that the minimal structure of GBP containing mitogenic activity is 2-23 GBP, whereas that with plasmatocyte spreading activity is 1-22 GBP, indicating the possibility that multifunctional properties of this peptide may be mediated by different forms of GBP receptors. In order to demonstrate this speculation, we analyzed GBP receptors in different types of cells by a competitive receptor binding assay with ^<125>I-GBP. The results showed that High Five cells contain two types of GBP receptors : high affinity (K_d=0.26) and low affinity (K_d=13.7), while hemocytes contain a single class of GBP receptor with a medium affinity (K_d=1.26). These results demonstrated a heterogeneity of GBP receptors at least in insect culture cells and hemocyte cells. Further, we identified two different types of GBP receptor candidates in hemocytes, approximately 60 kDa and 77 kDa proteins. The former protein does not contain any distinct domain structures other than one transmembrane domain. However, the latter protein contains one transmembrane domain, and ITAM (immunoreceptor tyrosine-based activation motif) and IgG like domains in the outer cell region and inner cell region, respectively. We further found that tyrosine residues in the cytoplasmic region of this molecule are phosphorylated by GBP. We are now trying to demonstrate whether GBP specifically binds to these cell membrane proteins.