Lipidic cubic phase crystallization of bacteriorhodopsin and cryotrapping of intermediates: towards resolving a revolving photocycle

Bacteriorhodopsin is a small retinal protein found in the membrane of the h alophilic bacterium Halobacterium salinarum, whose function is to pump prot ons across the cell membrane against an electrostatic potential, thus conve rting light into a proton-motive potential needed for the synthesis of ATP. Because of its relative simplicity, exceptional stability and the fundamen tal importance of vectorial proton pumping, bacteriorhodopsin has become on e of the most important model systems in the field of bioenergetics. Recent ly, a novel methodology to obtain well-diffracting crystals of membrane pro teins, utilizing membrane-like bicontinuous lipidic cubic phases, has been introduced, providing X-ray structures of bacteriorhodopsin and its photocy cle intermediates at ever higher resolution. We describe this methodology, the new insights provided by the higher resolution ground state structures, and review the mechanistic implications of the structural intermediates re ported to date. A detailed understanding of the mechanism of vectorial prot on transport across the membrane is thus emerging, helping to elucidate a n umber of fundamental issues in bioenergetics. (C) 2000 Elsevier Science B.V . All rights reserved.