Clathrin-mediated endocytosis of yolk protein in the Drosophila oocyte

The developing egg chamber of Drosophila accumulates yolk protein to utilize during embryonic development. Egg chamber development is divided in 14 different stages, yolk protein endocytosis occurs between stages 8-11. Yolk protein is present in hemolymph, and the ovaries are bathed in hemolymph. Yolk protein forms a complex with Yolkless for endocytosis in oocyte. Yolkless is an LDL receptor superfamily member, and its cytosolic tail contains the signal for internalization by clathrin-mediated endocytosis. The Yolkless cytosolic tail harbors an atypical FxNPxA sequence instead of the commonly found tyrosine-based sorting signal FxNPxY. The tyrosine-based sorting signals interact with PTB domain-containing proteins. In the present study, I show that PTB domain-containing phagocytic protein Ced-6 is expressed in the cortical region of oocyte of the egg chamber. Ced-6 harbors all the biochemical properties of clathrin-associated sorting proteins. Ced-6 can physically interact with clathrin, AP-2 and phosphoinositide(4,5)P2. The Ced-6 PTB domain interacts with the cytosolic tail of Yolkless and clusters Yolkless into clathrin-coated pit. This clustering leads to internalization of Yolkless by clathrin-mediated endocytosis. In conjunction with the FxNPxA signal, Yolkless cytosolic tail contains a functional, although atypical, dileucine signal. This atypical dileucine signal can physically interact with AP-2. This interaction leads to the clathrin-mediated endocytosis of Yolkless. Clathrin adaptor AP-2 and Ced-6 function redundantly in yolk protein uptake, as Ced-6-null flies accumulate yolk protein. Results from this study led to a model where both AP-2 and Ced-6 interact with Yolkless independently to ensure its internalization. Interaction of Ced-6 with clathrin and AP-2 leads to effective clustering of Yolkless for internalization by clathrin-mediated endocytosis.