Schonbaum C P, Perrino J J, Mahowald A P
University of Chicago, Department of Molecular Genetics and Cell Biology, Chicago, Illinois 60637, USA.
Mol Biol Cell. 2000 Feb;11(2):511-21. doi: 10.1091/mbc.11.2.511.
In many insects, development of the oocyte arrests temporarily just before vitellogenesis, the period when vitellogenins (yolk proteins) accumulate in the oocyte. Following hormonal and environmental cues, development of the oocyte resumes, and endocytosis of vitellogenins begins. An essential component of yolk uptake is the vitellogenin receptor. In this report, we describe the ovarian expression pattern and subcellular localization of the mRNA and protein encoded by the Drosophila melanogaster vitellogenin receptor gene yolkless (yl). yl RNA and protein are both expressed very early during the development of the oocyte, long before vitellogenesis begins. RNA in situ hybridization and lacZ reporter analyses show that yl RNA is synthesized by the germ line nurse cells and then transported to the oocyte. Yl protein is evenly distributed throughout the oocyte during the previtellogenic stages of oogenesis, demonstrating that the failure to take up yolk in these early stage oocyte is not due to the absence of the receptor. The transition to the vitellogenic stages is marked by the accumulation of yolk via clathrin-coated vesicles. After this transition, yolk protein receptor levels increase markedly at the cortex of the egg. Consistent with its role in yolk uptake, immunogold labeling of the receptor reveals Yl in endocytic structures at the cortex of wild-type vitellogenic oocytes. In addition, shortly after the inception of yolk uptake, we find multivesicular bodies where the yolk and receptor are distinctly partitioned. By the end of vitellogenesis, the receptor localizes predominantly to the cortex of the oocyte. However, during oogenesis in yl mutants that express full-length protein yet fail to incorporate yolk proteins, the receptor remains evenly distributed throughout the oocyte.
在许多昆虫中，卵母细胞的发育在卵黄发生（卵黄蛋白在卵母细胞中积累的时期）之前会暂时停滞。在激素和环境信号的作用下，卵母细胞的发育恢复，卵黄原蛋白的内吞作用开始。卵黄摄取的一个关键成分是卵黄原蛋白受体。在本报告中，我们描述了果蝇卵黄原蛋白受体基因无黄卵（yl）编码的mRNA和蛋白质在卵巢中的表达模式及亚细胞定位。yl RNA和蛋白质在卵母细胞发育的很早阶段就开始表达，远在卵黄发生开始之前。RNA原位杂交和lacZ报告基因分析表明，yl RNA由生殖系滋养细胞合成，然后转运到卵母细胞中。在卵子发生的卵黄发生前阶段，Yl蛋白均匀分布于整个卵母细胞，这表明这些早期卵母细胞未能摄取卵黄并非由于受体缺失。向卵黄发生阶段的转变以通过网格蛋白包被小泡积累卵黄为标志。在此转变之后，卵黄蛋白受体水平在卵母细胞皮层显著增加。与其在卵黄摄取中的作用一致，受体的免疫金标记显示野生型卵黄发生期卵母细胞皮层的内吞结构中有Yl。此外，在卵黄摄取开始后不久，我们发现了多囊泡体，其中卵黄和受体明显分隔。到卵黄发生结束时，受体主要定位于卵母细胞的皮层。然而，在表达全长蛋白但未能结合卵黄蛋白的yl突变体的卵子发生过程中，受体仍均匀分布于整个卵母细胞。
