Biogenesis of purple membrane: regulation of bacterio-opsin synthesis.

In the cell membrane of Halobacteria a photoreceptor protein, bacteriorhodopsin, forms a twodimensional hexagonal crystalline network within the lipid matrix [l-4] . This specialized domain of the membrane has been termed purple membrane [5,6]. Besides its attractiveness for bioenergetic studies, the purple membrane appears highly suited for studying the biogenesis of a membrane: Purple membrane contains only one protein species (bacterioopsin), one pigment (retinal) bound to bacteria-opsin and two main lipid species. Its biogenesis is inducible: A situation of low oxygen supply [7] and/or of nutrient exhaustion triggers the membrane synthesis. In a recent paper [8] it was shown that (1) the synthesis of both the membrane components bacterioopsin and retinal are strictly coordinated. Only inducing conditions turn on both biosynthetic pathways. (2) Another differentiated domain of the plasma membrane termed the ‘brown membrane’ is involved in the biogenesis of the purple membrane. This domain has a lower buouyant density than purple membrane, contains bacteriorhodopsin and a cytochrome b-type protein besides lipid and some minor protein species. (3) Newly formed bacterioopsin is initially incorporated in the brown membrane and can only ‘crystallize’ to form the purple membrane patches after reaction with retinal and a subsequent modification step.