Dr. Tom Rapoport explains that eukaryotic cells contain many membrane-bound organelles each of which has a characteristic shape and distinctive functions that are determined by specific proteins. Most proteins are made in the cytosol but must move to different cellular destinations.  Signal sequences on the proteins act as “zip codes” and direct protein sorting. Many proteins sort first to the endoplasmic reticulum (ER) before moving to other intracellular organelles or the plasma membrane. Rapoport explains that the Sec 61 channel in the ER membrane is key to protein sorting. Solving the structure of Sec 61 allowed Rapoport’s lab to understand how proteins are transported across the ER membrane and directed to their final destination.