Structural determinants of purple membrane assembly

The purple membrane is a two-dimensional crystalline lattice formed by bact eriorhodopsin and lipid molecules in the cytoplasmic membrane of Halobacter ium salinarum. High-resolution structural studies, in conjunction with deta iled knowledge of the lipid composition, make the purple membrane one of th e best models for elucidating the forces that are responsible for the assem bly and stability of integral membrane protein complexes. In this review, r ecent mutational efforts to identify the structural features of bacteriorho dopsin that determine its assembly in the purple membrane are discussed in the context of structural, calorimetric and reconstitution studies. Quantit ative evidence is presented that interactions between transmembrane helices of neighboring bacteriorhodopsin molecules contribute to purple membrane a ssembly. However, other specific interactions, particularly between bacteri orhodopsin and lipid molecules, may provide the major driving force for ass embly. Elucidating the molecular basis of protein-protein and protein-lipid interactions in the purple membrane may provide insights into the formatio n of integral membrane protein complexes in other systems. (C) 2000 Elsevie r Science B.V. All rights reserved.