Peripheral Membrane Proteins

Abstract

The founding principle behind structural biology is that form equals function. Nowhere is this relation more apparent than in proteins that operate in environments in which aqueous and lipophilic phases meet. This biphasic environment strongly influences both the structure and function of these proteins. Membrane proteins can be classified as integral or peripheral, depending upon the nature of the protein-membrane association. For proteins in the former category, the membrane is an integral part of their structures, which are greatly perturbed by disruption of the membrane by detergents. The extent of interaction with the lipid bilayer is usually obvious by simple inspection of the protein structure because of what is known about lipid physical chemistry, i.e., that there is a large energy cost to burying uncompensated polar or charged protein residues in a strongly hydrophobic milieu. For example, the membrane-embedded portions of transmembrane proteins, such as bacterial reaction centers and porins, are localized by the extensive hydrophobic regions, which are frequently bordered by aromatic side-chains, found on the surfaces of these proteins. For these transmembrane proteins, whose functional roles are to transfer energy or substances from one aqueous pool to another across a nonaqueous barrier, the membrane serves to organize the protein structure. In contrast to integral membrane proteins, those classified as peripheral can be released from their membrane attachment through gentler means, without disruption of either membrane or protein structure.