The innermost chorionic layer of Drosophila. I. The role of chorin octamers in the formation of a family of interdigitating crystalline plates.

The innermost chorionic layer (ICL) within egg shells of Drosophila melanogaster is composed of thin, abutting three-dimensional crystalline plates which form a closed, membrane-like sheath. Collectively, the crystals within the sheath appear to form a family of related three-dimensional crystals in space group C222; however, specimens prepared for electron microscopy are actually two-dimensional crystals in c222. The projected structures of the negatively stained crystals have been studied by minimal dose electron microscopy employing image reconstruction methods. Thin sections indicate that unit cells within the ICL are composed of paired layers; top and bottom layers are related by centrally located 2-fold axes, aligned parallel to the surface of the ICL. The most probable structural unit of the crystals is a tetramer of chorin dimers with a point group symmetry of 222, which is denoted a chorin octamer. Projection maps were computed from average transforms of two-dimensional crystals for delta (the primitive unit cell angle) equal to 84 degrees, 90 degrees and 97 degrees (+/- 1.5 degrees). The maps indicate that the molecular transitions responsible for the observed family of crystals involve concerted intramolecular rearrangements about molecular 2-fold axes. The significance in vivo of the family of crystals within the ICL is not known; however, structural considerations suggest that the observed polymorphism may reflect one facet of an intrinsic bonding flexibility of the ICL octamer that may play a role in the formation of interplate junctions and the assembly of a continuous closed sheath. The ICL may therefore serve as a structural bridge between the vitelline membrane-wax layer and the endochondrial floor, allowing the larva to shed the inner egg shell layers during hatching.