Different types of membrane proteins.

Some integral membrane proteins (a) have transmembrane segments that traverse the phospholipid bilayer. Such proteins usually have a series of ? helical domains that contain nonpolar (hydrophobic) amino acids, shown in inset (b). Other integral membrane proteins (c) are covalently linked to a fatty acid that can anchor the protein in the membrane interior. Peripheral membrane proteins are associated more weakly with a membrane region. Some peripheral proteins (d) associate with the hydrophilic region of an integral membrane protein, other peripheral proteins form ionic interactions with the polar head groups of lipids. Inset: The protein shown here is bacteriorhodopsin; it contains seven transmembrane segments in an ? helical structure (depicted as cylinders).