Membrane Protein Structural Classes
TM proteins can be classified into two basic types: a-helical and p-barrel proteins. a-helical membrane proteins form the major category of TM proteins and are present in all type of biological membranes, including bacterial outer membranes. They consist of one or more a-helices, each of which contains a stretch of hydrophobic amino acids, embedded in the membrane and linked to subsequent helices by extra-membranous loop regions. It is thought such proteins may have up to 20 TM helices allowing a diverse range of differing topologies. Loop regions are known to contain substructures including re-entrant loops—short a-helices that enter and exit the membrane on the same side—as well as amphipathic helices that lie parallel to the membrane plane, and globular domains. p-barrel TM proteins (TMBs) mainly consist of transmembrane p-strands that form a closed barrel in the membrane. Analysis of solved p-barrel 3D structures show that these proteins can consist of 8-26 p-strands arranged in an anti-parallel manner in the bacterial outer-membrane. Some TMBs also have large plug-domains and outer loops that can interact with the barrel region to control substrate transport.