RECEPTOR-MEDIATED ENDOCYTOSIS OF STORAGE PROTEINS BY THE FAT-BODY OF HELICOVERPA-ZEA

The selective uptake of storage proteins by the fat body of the corn e arworm Helicoverpa ten is mediated by a membrane-bound receptor protei n. In this study, the major storage proteins of this insect species, a rylphorin and very high density lipoprotein, were directly labeled wit h colloidal gold-particles of different size. After the fat body had b een incubated with the labeled storage proteins, the distribution of t hese proteins was examined by electron microscopy. Both storage protei ns were found at the extracellular side of coated pits and within coat ed vesicles. Moreover, fusion products of several coated vesicles such as endosomes and multivesicular bodies contained both proteins in the ir lumen. Ultimately, the proteins accumulated in electron-dense stora ge granules. Equal numbers of either storage protein were present in e ach organelle, supporting the notion that a single receptor mediates t he uptake of both proteins In contrast, only small numbers of gold-lab eled Immunoglobulin G molecules were found in the organelles, indicati ng that the protein uptake is specific for storage proteins. The resul ts show that storage protein uptake in this lepidopteran species occur s in a process of receptor-mediated endocytosis that is similar to the well-established uptake of specific proteins into mammalian tissues.