SECRETORY AND INTERNALIZATION PATHWAYS OF MOSQUITO YOLK PROTEIN PRECURSORS

The vitellogenic female fat body of the mosquito Aedes aegypti produce s three yolk protein precursors that are deposited in the yolk bodies of developing oocytes: vitellogenin, vitellogenic carboxypeptidase (VC P), and 44-kDa protein (44KP). We have used gold immunocytochemistry t o investigate the pathways of their secretion in fat body trophocytes and their internalization by oocytes. In fat body trophocytes, all thr ee yolk protein precursors are co-localized in the Golgi complex and s ecretory granules, indicating that they proceed simultaneously through the secretory pathway. The lysosomal system plays an important role i n the termination of vitellogenesis in mosquito trophocytes, by degrad ing biosynthetic organelles and secretory granules. At this time, VCP and 44KP are found together with vitellogenin in trophocyte autophagol ysosomes, suggesting that all three yolk protein precusors are redirec ted from the secretory to the lysosomal degradative pathway. Localizat ion 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 protei n precursors are found on the oocyte microvillus membrane, in coated v esicles, and early endosomes. They are observed intermixed with one an other in the late endosomes or in transitional yolk bodies. In mature yolk bodies, however, 44KP and VCP are segregated from vitellin, the c rystallized storage form of vitellogenin; 44KP and VCP reside in the n on-crystalline cortex, surrounding the vitellin core in nature yolk bo dies.