Role of Helix-Helix Interactions in Assembly of the Bacteriorhodopsin Lattice †

The purple membrane of Halobacterium salinarumis a two-dimensional lattice of lipids and the integral membrane protein bacteriorhodopsin (BR). To determine whether helix-helix interactions within the membrane core stabilize this complex, we substituted amino acid residues at the helix-helix interface between BR monomers and examined the assembly of the protein into the lattice. Lattice assembly was demonstrated to fit a cooperative self-assembly model that exhibits a critical concentration in vivo. Using this model as the basis for a quantitative assay of lattice stability, bulky substitutions at the helix - helix interface between BR monomers within the membrane core were shown to be destabilizing, probably due to steric clash. Ala substitutions of two residues at the helix -helix interface also reduced stability, suggesting that the side chains of these residues participate in favorable van der Waals packing interactions. However, the stabilizing interactions were restricted to a small region of the interface, and most of the substitutions had little effect. Thus, the contribution of helix -helix interactions to lattice stability appears limited, and favorable interactions between other regions of neighboring BR monomers or between BR and lipid molecules must also contribute. Many integral membrane proteins must assemble into stable oligomeric complexes to function properly. A long- standing challenge has been to identify and quantify the noncovalent interactions among lipids, proteins, and their prosthetic groups that drive the assembly of these complexes and maintain their stability (1-6). Due to the unique properties of the membrane, interactions that stabilize membrane protein oligomers may be significantly more complex than those that stabilize water-soluble proteins. These interactions remain poorly understood, in part because of the difficulty of measuring protein association in the lipid bilayer (4).