SEARCH Articles Figures Tables Fusion of a vesicle with the plasma membrane preserves the orientation of any integral proteins embedded in the vesicle bilayer. Initially, the amino terminal of the protein faces the lumen, or inner cavity, of such a vesicle. After fusion, the amino terminal is on the exterior surface of the plasma membrane. That the orientation of the protein has not been reversed can be perceived by noting that the other end of the molecule, the carboxyl terminal, is always immersed in the cytoplasm. The lumen of a vesicle and the outside of the cell are topologically equivalent. Re drawn and modified, with permission, from Lodish HF, Rothman JE [Publishers Imprint] [Publishers Imprint] [Publishers Imprint] [Publishers Imprint] [Publishers Imprint] [Publishers Imprint]