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.