The topography of the purple membrane

Abstract

THE purple membrane of Halobacterium halobium functions as a light-driven proton pump^1,2. It is about 45 Å thick³ and contains a single polypeptide chain comprising about 240 amino acids⁴. Retinal is covalently attached to a specific lysine residue⁴ and has been implicated in the absorption of incident light energy². The retinal lies at an angle of about 20° to the plane of the bilayer⁵. Seven protein rods running through the membrane, probably α-helices, have been detected by electron diffraction^6,7. Circular dichroism studies indicate a 75% helical content which is consistent with a seven-helix model⁸. A structure containing seven such α-helices requires about 210 amino acids to cross a membrane 45 Å thick seven times, thus implying that most of the polypeptide chain is buried in the lipid bilayer. Digestion of the membrane with proteolytic enzymes supports this view and only three parts of the polypeptide chain are apparently exposed. Here, we show by sequence analysis of digestion products that a section corresponding to the N-terminal seven residues is removed by pronase and proteinase K.