A new family of growth factors produced by the fat body and active on Drosophila imaginal disc cells

By fractionating conditioned medium (CM) from Drosophila imaginal disc cell cultures, we have identified a family of Imaginal Disc Growth Factors (IDG Fs), which are the first polypeptide growth factors to be reported from inv ertebrates. The active fraction from CM, as well as recombinant IDGFs, coop erate with insulin to stimulate the proliferation, polarization and motilit y of imaginal disc cells. The IDGF family in Drosophila includes at least f ive members, three of which are encoded by three genes in a tight cluster. The proteins are structurally related to chitinases, but they show an amino acid substitution that is known to abrogate catalytic activity. It therefo re seems likely that they have evolved from chitinases but acquired a new g rowth-promoting function. The IDGF genes are expressed most strongly in the embryonic yolk cells and in the fat body of the embryo and larva. The pred icted molecular structure, expression patterns, and mitogenic activity of t hese proteins suggest that they are secreted and transported to target tiss ues via the hemolymph, However, the genes are also expressed in embryonic e pithelia in association with invagination movements, so the proteins may ha ve local as well as systemic functions, Similar proteins are found in mamma ls and may constitute a novel class of growth factors.