Role of helix-helix interactions in assembly of the bacteriorhodopsin lattice

The purple membrane of Halobacterium salinarum is a two-dimensional lattice of lipids and the integral membrane protein bacteriorhodopsin (BR). To det ermine whether helix-helix interactions within the membrane core stabilize this complex, we substituted amino acid residues at the helix-helix interfa ce between BR monomers and examined the assembly of the protein into the la ttice. 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 substitution s at the helix-helix interface between BR monomers within the membrane core were shown to be destabilizing, probably due to steric clash. Ala substitu tions 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 w ere restricted to a small region of the interface, and most of the substitu tions had little effect. Thus, the contribution of helix-helix interactions to lattice stability appears limited, and favorable interactions between o ther regions of neighboring BR monomers or between BR and lipid molecules m ust also contribute.