Experimental modifications of an insect vitellin affect its structure and its uptake by oocytes

The 18S and 33S vitellins (Vts) of Blattella germanica were subjected to periodate oxidation and digestions with α‐mannosidase, endo‐β‐Nacetylglucosaminidase H (endo‐H), and trypsin to study their effects on Vt structure and function. Periodate oxidation caused 33S Vt to dissociate to a form that cosedimented with 18S Vt upon glycerol gradient centrifugation but had no effect on the sedimentation of 18S Vt. This result implicates the oligosaccharides in stabilization of the 33S structure. Incubation of 18S and 33S Vts with α‐mannosidase and endo‐H revealed that the oligosaccharides of both Vts are largely shielded from attack by both glycosidases. However, the carbohydrate of 18S Vt was 3 to 5 times more susceptible to both enzymes, suggesting that the 18S to 33S transition results in decreased accessibility of the oligosaccharides to both glycosidases. Short‐term exposure of 18S and 33S Vts to trypsin resulted in limited hydrolysis; the M r 102,000 subunit of each form was cleaved with an M r 79,000 peptide as a major product. However, the sedimentation properties of the Vts and their relative susceptibilities to α‐mannosidase were unchanged; therefore while the M r 102,000 subunit of the Vt is vulnerable to trypsin, it retains its higher‐order structure after limited digestion. Endocytosis of radiolabelled 18S Vt by oocytes in vivo decreased about 15‐fold after its modification by periodate and sixfold after treatment with α‐mannosidase. Limited trypsin digestion also severely diminished its uptake. All injected radioactivity of unmodified 18S and 33S Vts could be recovered from either the hemolymph or ovaries of recipient females. However, modified Vts were taken up from the hemolymph primarily by cells of the pericardium and the fat body, suggesting that these organs participate in a clearance mechanism that recognizes “damaged” Vt molecules.