A New Protein in Insects

The cochineal insect, Dactylopius confusus, a scale insect, is commercially important because it produces a nontoxic red dye, carminic acid. An investigation into its biology has produced fascinating details as well as some intriguing questions. One question involves specific proteins that are necessary for the insect's reproduction.

All egg-laying insects must package their eggs with the necessary materials for the production of a whole new insect. The most abundant protein stored in eggs, a specialized yolk protein, is called vitellin. The precursor to vitellin, vitellogenin, is synthesized in the fat body of the female (similar to humans' liver and fatty tissue) and then taken up by the egg through a process called receptor-mediated endocytosis. When vitellogenin binds to receptors on the surface of the egg, coated pits and then coated vesicles are formed around the receptor-bound vitellogenin, which is drawn into the oocyte. D. confusus does not produce vitellin, but we have isolated a different yolk protein that can form tremendously large complexes. Electron microscopy of negatively stained preparations of the cochineal yolk protein (CYP) has shown that CYP is an unusually large, corkscrew-shaped protein. These large molecules of CYP are highly organized polymers of small subunits. The negative staining involves drying of the sample and therefore can cause distortion. However, the corkscrew structure was verified by cold stage electron microscopy, which involves quick freezing of a sample in order to preserve the protein in its native state.

CYP found inside eggs has been measured at up to several hundred nanometers long. This enormous size appears to preclude receptor-mediated endocytosis; coated vesicles are just 150nm-190nm in diameter. Are smaller polymers or the subunits endocytosed by the egg and then assembled in the large units? If so, why are the large polymers also found in hemolymph samples taken from females? We do not know how these long "corkscrew" proteins are assembled from the smaller subunits or what forces hold them together.

We are intrigued by this unusual structure and would like to understand the chemical forces that arrange the subunits in this peculiar arrangement. Our future experiments will therefore focus on taking these structures apart and reassembling them and on finding out how CYP is taken up into the oocyte.