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 alpha- and beta-yolk spheres. By applying biochemical and transmission electron microscopy (TEM) immunogold techniques we found that the beta-yolk spheres contain mainly polysaccharides, while the three main yolk proteins (YPs) are stored in the alpha-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 alpha-yolk spheres of the mature oocyte. Our subsequent two-dimensional reconstruction analysis revealed that the unit cell of the crystal is about 113 Angstrom x 113 Angstrom. Assuming that the repeating unit is a cylinder of about 110 Angstrom in length and 25 Angstrom in diameter this cylinder would then have a volume of about 50,000 cubic Angstrom, 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 alpha-yolk spheres constituent molecules and provides direct evidence for the "natural" crystallization, and therefore the efficient packaging, of the YPs during oogenesis.
Papassideri
Issidora S
IS
Department of Cell Biology and Biophysics, Athens University, Panepistimiopolis, Zografou, Athens 15784, Greece. ipapasid@biol.uoa.gr
Trougakos
Ioannis P
IP
Leonard
Kevin R
KR
Margaritis
Lukas H
LH
D000818:Animals; D004331:Drosophila melanogaster; D004527:Egg Proteins / Q000378:metabolism; D004530:Egg Yolk / Q000648:ultrastructure*; D005260:Female; D005614:Freeze Fracturing; D007091:Image Processing, Computer-Assisted; D046529:Microscopy, Electron, Transmission; D009865:Oocytes / Q000648:ultrastructure*
D004527:Egg Proteins