The picture of the channel that emerges is one of a dynamic, fluid structure that appears and disappears on demand. By contrast, most other channel molecules are permanently lodged in the membrane. Even more unusual, Rapoport says, is that the channel can open in two dimensions to deliver secreted and membrane-bound proteins properly in different ways. For example, insulin passes all the way through the ER membrane into the lumen of the ER and eventually gets spilled into the bloodstream. However, the insulin receptor slides only halfway into the ER membrane.
Protein translocation is so fundamental, it is very similar in all living creatures.
Even bacteria use a protein translocation system for secretion. In an upcoming study, Rapoport has collaborated with Hidde Ploegh at MIT to learn how a virus evades the body's attempt to alert killer T cells to its presence. It turns out, says Rapoport, that the virus probably pushes the body's flags, the MHC proteins, out of the ER back into the cytosol, where they are promptly degraded, never getting to display evidence of the infection to the immune system. Additional authors of the Cell article are Kent Matlack, Berit Jungnickel and Kathrin Plath, all of Harvard Medical School, Dorit Hanein and Christopher Akey of Boston University, Ken Miller of Brown University, and Kai-Uwe Kalies of the Max Delbrueck Center for Molecular Medicine in Berlin.