Ultrastructural and genetic studies were carried out on the fat body of a female sterile mutant fs(1)1163 to ascertain why yolk protein 1 (YP1) is not secreted from this tissue. Earlier molecular studies demonstrated that (a) normally yolk protein is synthesized in the fat body, secreted into the hemolymph and taken up by the ovary, (b) the 1163 mutation causes a single amino acid substitution in YP1, and (c) females homozygous for the mutation, or heterozygous females raised at 29 degrees C, retain YP1 in the fat body. Ultrastructural analysis in this paper shows that the fat body of these females contains masses of electron-dense material deposited in the subbasement membrane space. This subbasement membrane material (SBMM), which occasionally has a crystalline-like, fibrous component, is found in females whose genotypes include at least one copy of the mutant 1163 gene. These strains include a deletion strain that is hemizygous for the 1163 gene and two strains that are transgenic for the mutant gene. Immunogold studies indicate that SBMM contains yolk protein. We propose that the mutant protein is secreted into the subbasement membrane space, but because of the amino acid substitution in YP1, the oligomers containing YP1 condense into SBMM, which cannot penetrate the basement membrane. The similarity of SBMM and deoxyhemoglobin S fibers is discussed.

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