| Abstract: | The three yolk polypeptides (YPs) of Drosophila are synthesized and secreted by female fat body and ovarian follicle cells, sequestered by pinocytosis into oocytes, and finally deposited into yolk granules. The biosynthesis of the YPs was studied using two-dimensional gels. Labeling the YPs with 35S]-cysteine, an amino acid found only near the amino terminus of YP1 and YP2, showed that an amino terminal peptide is removed from YP1 and YP2 shortly after or during translation. Intermediates in YP biosynthesis corresponding in electrophoretic mobility to pancreatic membrane-processed primary translation products were also detected in a 5-min pulse label with 35S]-methionine. Genetic variants that alter YP structure were used to identify which YP precursor comes from which Yp gene. Pulse labeling with 35S]-methionine revealed that all three YPs becomes more negatively charged, that YP1 and YP2 become heterogeneously charged, and that YP1 gains in apparent molecular weight within 15 min after translation. Injecting female flies with radioiabeled sugars or orthophosphate revealed that the YPs are glycosylated and phosphorylated. Treating hemolymph proteins with phosphatase showed that phosphorylation is responsible for much of the change in charge and increase in molecular weight of the maturing YPs. These experiments with wild-type flies provide a basis for the analysis of mutations at the Yp genes which alter the structure of individual YPs. |
