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

Abstract

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 Δ of 90 °, has been determined from a complete tilt series of a negatively contrasted specimen at a resolution of 25 Å. 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 α and β. The chorin octamer is composed of two classes of bent dimers, which make intramolecular contacts at the top and bottom of the molecule. The α-dimers are curved outwards away from the crystallographic 2-fold axis, while the β-dimers are curved towards the molecular center. In addition, lattice contacts are formed primarily by β-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 × 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.