Principles of membrane protein structure

Abstract

This chapter describes the structure of integral membrane proteins (IMPs)—in particular, on their transmembrane (TM) domains. Many membrane proteins also possess extensive extramembranous domains. There are two major types of membrane proteins—integral and peripheral. Integral proteins interact strongly with membranes as a result of their polypeptide chains spanning the bilayer one or more times, whereas peripheral proteins interact more weakly with the membrane surface. Peripheral proteins may form electrostatic interactions with phospholipid headgroups or may bind to integral proteins. Peripheral proteins are thought to obey the same structural principles as soluble proteins. The key structural feature of IMPs is that their polypeptide chains span lipid bilayers, thus excluding those regions of the chains from contact with water molecules. This has two structural consequences: (1) membrane-spanning regions of polypeptide chains adopt a defined secondary structure, often α-helical; and (2) membrane-spanning sequences are predominantly hydrophobic. Many IMPs exhibit more complex topologies, their polypeptide chain crossing the bilayer several times. In such molecules, a considerable fraction of the mass of the protein may be located within the bilayer. The most common class of such IMPs is those with an all-α topology, in which several TM helices span the bilayer. This class of IMPs may be extended to include microassemblies of single TM helix proteins, channel-forming peptides (CFPs), and membrane-active toxins, which form transbilayer pores by intramembranous association of TM helices to generate parallel bundles.