Crystalline yolk spheroids in Drosophila melanogaster oocyte: Freeze fracture and two-dimensional reconstruction analysis

The major sites of energy storage during oogenesis in the Drosophila melanogaster oocyte are the α- and β-yolk spheres. By applying biochemical and transmission electron microscopy (TEM) immunogold techniques we found that the β-yolk spheres contain mainly polysaccharides, while the three main yolk proteins (YPs) are stored in the α-yolk spheres of the developing oocyte. Moreover, by using high-resolution TEM of freeze fractured or cryosectioned follicles, we identified the existence of crystalline structures within the α-yolk spheres of the mature oocyte. Our subsequent two-dimensional reconstruction analysis revealed that the unit cell of the crystal is about 113 Å×113 Å. Assuming that the repeating unit is a cylinder of about 110 Å in length and 25 Å in diameter this cylinder would then have a volume of about 50,000 cubic Å, which corresponds to about 40 kDa of protein. This size fits quite well with the known molecular weight of about 40-45 kDa for each of the three D. melanogaster YPs. Overall, our study identifies for the first time the supramolecular arrangement of the α-yolk spheres constituent molecules and provides direct evidence for the “natural” crystallization, and therefore the efficient packaging, of the YPs during oogenesis.