Synthesis of yolk proteins in Drosophila melanogaster

Abstract

Three yolk proteins, YP1, YP2 and YP3, have been found in Drosophila melanogaster. The polypeptides are present in the egg in large aggregates of very high molecular weight. Proteins of similar molecular weight to some of the yolk proteins are present in other tissues in male and female adults and larvae, but immunological techniques have shown that these are not identical to the yolk proteins. In vivo labelling experiments indicate that the yolk proteins are present only in the female fat body, haemolymph and ovary, whilst in vitro labelling experiments show that the fat body is the site of yolk proteins are where up to 12% of the protein made in a six hour labelling period may be yolk protein. The yolk proteins are secreted by the fat body in the form in which they are accumulated by the ovary rather than in precursor form. There is no evidence for the existence of a protein precursor to the yolk proteins during their synthesis in the fat body although the possibility of a small pool of precursor which is rapidly turning over cannot be eliminated.