Expression and changing distribution of the human transferrin receptor in developing Drosophila oocytes and embryos.

In order to understand better the membrane systems in a developing Drosophila oocyte, the human transferrin receptor has been expressed there. This was achieved using the armadillo promoter combined with K10 or oskar trailer sequences; these enable the messenger RNA to be transcribed in nurse cells and then transported to, and translated in, oocytes. This is the first exogenous protein to be expressed in oocytes. At stage 8, the transferrin receptors are mainly concentrated towards the posterior pole of the oocyte and are associated with large cytoplasmic vesicles; when combined with the shibire mutation the transferrin receptors are transferred to the oolemma, demonstrating that they participate in an endocytic cycle. At stage 10, the transferrin receptors are localised either to the anterior margin of the oocyte or to the posterior pole, depending on where the mRNA is located. In newly laid eggs, all the transferrin receptors are found in large cytoplasmic vesicles. The results reveal remarkable sorting processes which occur as oocytes mature and show that ring canals, which separate the oolemma from nurse cell plasma membranes, act as barriers to prevent components in these two compartments from intermixing.