Postendocytic vitellin processing in ovarian follicles of the stick insect Carausius morosus (Br.)

Newly laid eggs of the stick insect Carausius morosus contain two native vitellins (Vit A and Vit B). Under denaturing conditions, these vitellins resolved into 3 (A1, A2, and A3) and 2 (B1 and B2) polypeptides. All of these polypeptides had counterparts in the female hemolymph from which they were shown to be derived by in vivo labelling. During ovarian development, the 2 vitellins changed both in charge and polypeptide composition. In EV and LV follicles, Vit A resolved into 4 distinct vitellin polypeptides (A0, A1, A2 and A3). Using a panel of monoclonal antibodies, polypeptide A0 proved to be immunologically related to polypeptide A2. In follicles about to begin choriongenesis, polypeptide A3 was gradually replaced by a lower Mr polypeptide. Over the same time period, polypeptide B1 changed in charge, but not in Mr. To confirm the existence of a polypeptide processing in C. morosus, ovarian follicles of different developmental stages were exposed in vivo to [35S]-methionine from 6 to 72 h. Data showed that A0 and B1 were the polypeptides most heavily labelled after short time exposures to the radioisotope. Polypeptides B2 and A3 were also labelled to some extent. With progressively longer exposures, polypeptides A1 and A2 also became labelled. In vivo exposure to [3H]-GlcNAc caused all vitellin polypeptides to become heavily labelled. Autoradiographic analysis of ovarian follicles labelled this way showed that, during development, radioactivity was gradually transferred from newly formed yolk spheres in the cortical ooplasm to the central ooplasm. Data were interpreted as suggesting a causal relationship between polypeptide processing and progressive yolk sphere fusion to yield the central ooplasm.