The orientation of halorhodopsin in the cell membrane of halobacteria.

The orientation of the light-driven chloride pump, halorhodopsin, in the membrane was determined using antibodies directed against a synthetic peptide which represents the C-terminal segment of the protein. Antibodies against this decapeptide did not bind to right-side-out cell vesicles. Partial inversion by sonication or lysis under low salt conditions exposed this COOH-terminal antigenic site. Antibody binding was removed by preincubation with the decapeptide. The COOH terminus of the molecule is therefore located on the cytoplasmic surface of the membrane.