Isolation and characterization of the retinal-binding component of halorhodopsin

Halorhodopsin (HR) was reconstituted in cell vesicles prepared from Halobacterium halobium strain L-07 by addition of tritium-labelled retinal and subsequently reduced with cyanoborohydride. Lysis of the labelled vesicles in water and dissolution of the cell membranes with 4% SDS allowed the purification of the retinyl protein (RP) by a 3-step procedure. Gel filtration on AcA-44 ultrogel was followed by chromatography on hydroxylapatite and preparative SDS-polyacrylamide gel electrophoresis. This procedure yielded material which migrated as a single band of an apparent mol. wt. of 25 000 on analytical SDS-polyacrylamide gels. The purification was 400-fold with an overall yield of 15%. Not only the mol. wts. but also the amino acid compositions of the RPs from bacteriorhodopsin (BR) and HR are very similar. Polyclonal antibodies against BR and HR did not, however, crossreact. When the two RPs were partially digested with staphylococcal V8 protease the proteolytic pattern of the retinyl peptides was similar, but not identical: two extra peptides are present in BR. The same kind of differences were found in the h.p.l.c. elution profiles of retinyl peptides produced by subtilisin digestion. Therefore, the two proteins must be different gene products and not modification products of one and the same protein.