ACCURATE PROCESSING AND SECRETION IN THE BACULOVIRUS EXPRESSION SYSTEM OF AN ERYTHROID-CELL-STIMULATING FACTOR CONSISTING OF A CHIMERA OF INSULIN-LIKE GROWTH-FACTOR-II AND AN INSECT INSULIN-LIKE PEPTIDE

A synthetic gene encoding the signal peptide and the N-terminal sequen ce of bombyxin, an insect insulin like peptide, and the 58 amino acids of the C-terminal sequence of human insulin-like growth factor II (IG F II) has been expressed using the baculovirus system. This synthetic chimaera was obtained by amplification of four overlapping oligonucleo tides using Taq polymerase and cloning into the transfer vector pBlueb ac. The construct was integrated by homologous recombination into the Autographa californica nuclear polyhedrosis genome. Spodoptera frugipe rda Sf9 insect cells infected with the recombinant baculovirus secrete d an accurately processed peptide consisting of the ten N-terminal ami no acids of bombyxin and the 58 C-terminal amino acids of IGF II, The N-terminal glutamine of bombyxin was changed to asparagine to facilita te sequencing of the synthetic peptide. The chimaera was five times mo re potent than human recombinant IGF II in its capacity to stimulate t hymidine incorporation into erythroid cells of fetal bovine liver in a serum-free medium. It stimulated erythroid colony formation in the pr esence of 2 munits/ml erythropoietin in cells cultured over a monolaye r of stromal cells of fetal liver. Artificial chimaeras as described h ere may prove useful for the production of insulin, IGF I and other pe ptides as secreted proteins in insect cells.