Structure of chitin-protein complexes: Ovipositor of the ichneumon fly Megarhyssa

Abstract

A model has been developed for the three-dimensional arrangement of the chitin fibrils and matrix proteins in a typical chitin-protein complex from insect cuticles. This model is based on the X-ray fiber diagrams of ichneumon fly ( Megarhyssa) ovipositor, which is a highly oriented example of one of the most common types of structure in chitin-protein systems. The basic structural unit in the complex is a microfibril with a diameter of 7.25 nm, consisting of a crystalline chitin core surrounded by a sheath of ordered protein. Small and wide angle equatorial reflections show that these fibrils are hexagonally packed, as has been reported in electron microscope studies, and the small angle intensities are in good agreement with the squared Fourier transform of a solid cylinder. The layer-line spacings indicate that the protein repeat along the fibril is 3.06 nm, which corresponds to six. N-acetyl glucosamine residues of the chitin chains. The relative intensities on the layer-lines suggest that the protein consists of subunits arranged in a 6 1 helix around the crystalline chitin core. From the positions of the off-meridional maxima we estimate that the radius of the protein helix is ~2.5 nm, which is close to the center of the protein sheath. Finally, splitting and fanning of the layer-lines indicate that the chitin-protein microfibril has a longer axial repeat of 15.3 nm, possibly due to super coiling.