The innermost chorionic layer of Drosophila. II. Three-dimensional structure determination of the 90 degrees crystal form by electron microscopy.

The innermost chorionic layer (ICL) within egg shells of Drosophila is composed of a family of related, thin three-dimensional crystals that form a continuous sheath encapsulating the egg shell lumen. Junctions formed by interdigitating lattices play a central role in the construction of this macroscopic assembly. The three-dimensional structure of a two-dimensional crystal isolated from the ICL, with a primitive lattice angle delta of 90 degrees, has been determined from a complete tilt series of a negatively contrasted specimen at a resolution of 25 A. Inspection of the three-dimensional transform after data merging revealed that the space group is c222 and this symmetry was employed to generate a three-dimensional structure. The basic structural unit of the ICL is an octamer, described formally as a tetramer of dimers with point group symmetry 222. There are two classes of dimer in the octamer designated alpha and beta. The chorin octamer is composed of two classes of bent dimers, which make intramolecular contacts at the top and bottom of the molecule. The alpha-dimers are curved outwards away from the crystallographic 2-fold axis, while the beta-dimers are curved towards the molecular center. In addition, lattice contacts are formed primarily by beta-chorin dimers at both the top and bottom surfaces of the unit cell. The molecular weight of a chorin octamer determined from the analysis is about 6 X 10(5). The conformation of the chorin octamer determined here suggests that permutations of a basic molecular mechanism may be adequate to explain both the observed lattice polymorphisms of the ICL and the formation of interplate junctions necessary for the assembly of the macroscopic sheath.