Secretory and internalization pathways of mosquito yolk protein precursors

The vitellogenic female fat body of the mosquito Aedes aegypti produces three yolk protein precursors that are deposited in the yolk bodies of developing oocytes: vitellogenin, vitellogenic carboxypeptidase (VCP), and 44-kDa protein (44KP). We have used gold immunocytochemistry to investigate the pathways of their secretion in fat body trophocytes and their internalization by oocytes. In fat body trophocytes, all three yolk protein precursors are co-localized in the Golgi complex and secretory granules, indicating that they proceed simultaneously through the secretory pathway. The lysosomal system plays an important role in the termination of vitellogenesis in mosquito trophocytes, by degrading biosynthetic organelles and secretory granules. At this time, VCP and 44KP are found together with vitellogenin in trophocyte autophagolysosomes, suggesting that all three yolk protein precusors are redirected from the secretory to the lysosomal degradative pathway. Localization of VCP and 44KP in developing mosquito oocytes clearly shows that the internalization of these yolk protein precursors by oocytes occurs via the same endocytotic route as vitellogenin: all three yolk protein precursors are found on the oocyte microvillus membrane, in coated vesicles, and early endosomes. They are observed intermixed with one another in the late endosomes or in transitional yolk bodies. In mature yolk bodies, however, 44KP and VCP are segregated from vitellin, the crystallized storage form of vitellogenin; 44KP and VCP reside in the non-crystalline cortex, surrounding the vitellin core in nature yolk bodies.