Processing of pro-vitellogenin in insect fat body: a role for high-mannose oligosaccharide.

Several discrete events were resolved in the processing of vitellogenin in Blattella germanica. Using tunicamycin to inhibit the synthesis of high-mannose oligosaccharide, a high molecular weight pro-vitellogenin peptide (apo-proVG, Mr 215,000) was identified in fat body. Dosages of tunicamycin which inhibited glycosylation of vitellogenin by 98% inhibited its synthesis by as much as 59%, yet led to an intracellular accumulation of apo-proVG. Reversibility and dose dependency of these effects on vitellogenin synthesis, glycosylation, proteolytic processing, and secretion were demonstrated. In control insects, glycosylation of apo-proVG yielded a Mr 240,000 pro-vitellogenin peptide (proVG). FITC-Concanavalin A bound to purified proVG but not to apo-proVG, thus confirming an absence of high-mannose oligosaccharide in the apo-protein. Following its glycosylation, proVG was processed rapidly in fat body to Mr 160,000 (VG160) and Mr 102,000 (VG102) peptides which subsequently were secreted into hemolymph. After uptake into developing oocytes, the VG160 peptide was processed further prior to chorionation, yielding subunits of Mr 95,000 and 50,000. Uniqueness of the peptides of mature vitellin (Mr 102,000, 95,000, and 50,000) was indicated by comparison of the CNBr fragments of each purified subunit. Staining of CNBr fragments with FITC-Concanavalin A also indicated that high-mannose oligosaccharides are attached at one or more sites within each vitellin subunit. Resolution of the substructure of this insect vitellin and identification of events involved in the processing and secretion of its fat body apo-protein provide a basis for further study of the assembly and transport of vitellogenin, its packaging in eggs, and utilization during embryogenesis.