Transmembrane proteins: Two structural types can be distinguished based on their transmembrane secondary structure.
Two classes of membrane proteins are currently known, that are characterized by the structure of their transmembrane domain. α-helical proteins, for example bacteriorhodopsin, form a bundle of transmembrane helices that span the phospholipid bilayer. In Matrix Porin, antiparallel β-strands span the membrane and form a barrel like structure. The outer membrane protein A (OmpA, which I use as a model to study membrane protein folding) belongs to the β-barrel class of membrane proteins. For more β-barrel membrane protein structures, click here. For more α-helical bundle membrane proteins click here