An autoradiographic analysis of vitellogenin synthesis and secretion in the fat body of the stick insect Bacillus rossius

Abstract

The fat body of the stick insect Bacillus rossius was studied with a view to clarifying the metabolic pathway leading to secretion of vitellogenin (VG). Electrophoretic analysis of ovarian follicles and hemolymph from egg-laying females showed that the two tissues shared a common polypeptide composition consisting of five major polypeptides with molecular weights ranging from 60–180 Kd. Following in vivo exposure to [ 35S]-methionine for up to 24 h, these polypeptides were labeled in a stage- and time-dependent manner, suggesting that they were transferred from the hemolymph to the oocyte during vitellogenesis. Fat body pulse-labeled with [ 35S]-methionine for up to 240 min and immunoprecipitated with an anti-yolk serum was labeled only in a fraction containing high molecular weight polypeptides. We presume these polypeptides to be VG precursors bearing a precursor-product relationship with the five major polypeptides of the hemolymph and developing ovarian follicles. Fat body exposed in vivo to [ 3H]-leucine for time intervals ranging from 20–240 min were processed for EM autoradiography. The results of this analysis showed that incorporated radioactivity was progressively transferred from the endoplasmic reticulum to the Golgi apparatus and from there to the composite granules. The data provided in this study are consonant with previous findings by which composite granules were shown to contain two compartments differing both in content and origin. In addition, the autoradiographical data of in vivo labeled fat body demonstrate that only the material partitioned into the electron-dense compartment of these granules is exocytosed.