A yolk protein mutant leads to defects in the secretion machinery of Drosophila melanogaster

The three yolk proteins of Drosophila melanogaster are synthesized in the f at body and ovarian follicle cells. A mutation in yolk protein 3, YP3(S1), has been described in which the leader sequence is not cleaved from the pro tein. We describe here ultrastuctural and molecular studies on the YP3(S1) mutant and show that the mutant protein enters the secretory pathway and fo rms precipitates, often as electron dense material in excessive elaboration s of the plasma membrane. Females homozygous for YP3(S1) lay fewer eggs tha n wild type flies and these embryos are less viable. The abnormal ultrastru cture of the yolk spheres observed suggests that whilst YP3 is not complete ly essential for viability, it is required for normal yolk sphere morphogen esis.