Figure 2.

(A) A model extended from that proposed by Johnstone et al. (Johnstone, 2005). Reticulocytes mature through the release of TfR-containing exosomes from MVEs. Clathrin coated vesicles and lipid rafts have been added to the original scenario, to show that TfR is recruited at clathrin-coated pits and endocytosed in clathrin-coated vesicles (1) that do not contain membrane rafts nor band 3, GPC, or GPA. Parallel routes of endocytosis must exist that deliver membrane raft components to the MVB while, again, excluding band 3, GPC and GPA (2). After shedding of the clathrin coat (3) from clathrin-coated vesicles, all vesicles converge and fuse (4) in a single early endosome, which is soon converted into a MVB with the endovesiculation of ILVs that contain both membrane raft components and TfR (5). The membrane of the MVB becomes thus depleted of TfR and membrane raft constituents, including cholesterol and sphingolipids, The MVB eventually fuses with the plasma membrane and releases the ILVs that are now defined as exosomes. The bulky structure of the spectrin skeleton is depicted as separated from the scenario where this vesicular trafficking occurs. In (B) a different pathway is proposed whereby band 3, GPC, and GPA can reach the MVB membrane coming from, for instance, clathrin-coated vesicles, but then are not packaged into ILVs and exosomes because of their inability to partition to the raft phase. Band 3, GPC, and GPA are therefore returned to the plasma membrane with the fusion of the MVB with it. See text for additional details.