Studies on insect fibrous proteins: the structural protein of the ootheca in the praying mantis, Sphodromantis centralis Rehn

Histological studies have been made on the left colleterial gland which secretes the alpha-type structural protein in the ootheca of Sphodromantis centralis. Three distinct types of secretory cell are present in the gland wall, and each has a complex apical 'end-apparatus' composed of closely packed canalicules radiating from a central cavity. Structural protein is synthesized in the cytoplasm, passed through the canalicules into the cavity of the end-apparatus, and extruded into the lumen as strongly birefringent, apparently membrane-free globule;. At this stage the globules consist of numerous concentric layers of long, parallel fibrils, 0.05-0.1 mu m in diameter. In each successive layer the fibril direction is rotated through an angle of about 18 degrees about an axis perpendicular to the layers, in the manner of cholesteric phase systems. When sectioned, the globules reveal a regular 'lamellated' structure similar to that apparent in sections of some insect and crustacean cuticles. A three-dimensional model has been constructed to illustrate this phenomenon. During ootheca formation fibrils become transformed into thin 'crystalline' ribbons, about 15 pm long, 1-2 mu m wide and 200-300 angstrom thick. Very regular diamond-shaped ribbons may be obtained in vitro by mixing structural protein globules from dissected left glands with optimum concentrations (0.005-0.025 M) of calcium chloride in unbuffered solution. Experiments suggest that calcium ions play an important part in the natural process of ribbon formation.