A novel cell-free translation/glycosylation system prepared from insect cells

A cell-free translation/glycosylation system derived from lepidopteran (Sf21) cells, which are widely used to express high yields of foreign active proteins that have post-translational modifications, was constructed. The insect cell extract was prepared using a Mini-Bomb cell disruption chamber by nitrogen pressure treatment, which stably retains translational and post-translational components. The gp120 mRNA was transcribed from the human immunodeficiency virus type-1 envelope glycoprotein gp120 gene with T7 RNA polymerase. When the gp120 mRNA was translated in the insect cell-free system, gp120 having a molecular mass of 100 kDa was detected by Western blot analysis. Synthesized gp120 and gp120 expressed in the intracellular fraction of recombinant-baculovirus-infected Sf21 cells had the same molecular mass, and they both had reduced mobility compared with gp120 secreted by recombinant baculovirus-infected Sf21 cells. In contrast, the 56-kDa gp120 protein, which corresponds to the polypeptide backbone of gp120, was synthesized in wheat germ and rabbit reticulocyte systems. The molecular mass of synthesized gp120 decreased from 100 kDa to 61 kDa after endoglycosidase H treatment, indicating that synthesized gp120 had been glycosylated with N-linked oligosaccharides. Furthermore, glycosylated gp120 was bound to human CD4 molecules expressed on the surface of quail cells. These results revealed that the insect cell-free system can synthesize gp120 that is folded in the proper conformation to provide a CD4-binding domain.