The structures of integral membrane proteins from 3 different classes have now been determined. The first class is that of proteins embedded in the membrane by 1 or more transmembrane-spanning alpha-helices. The second class is represented by class of proteins that form pores through the membrane, and are formed by 16 or 18 beta strands in a barrel arrangement. The third class is represented by a group of enzymes that are anchored to a single leaflet of the membrane via several amphipathic alpha-helices.