title: Refolding and proton pumping activity of a polyethylene glycol-bacteriorhodopsin water-soluble conjugate.
abstract: Bacteriorhodopsin (BR), from the purple membrane (PM) of Halobacterium halobium, was chemically modified with methoxypolyethylene glycol (m-PEG; molecular weight = 5,000 Da) succinimidyl carbonate. The polyethylene glycol-bacteriorhodopsin (m-PEG-SC-BR33) conjugate, containing one polyethylene glycol chain, was water soluble. The secondary structure of the conjugate in water appeared partially denatured, but was shown to contain alpha-helical segments by circular dichroism spectroscopy. The isolated bacteriorhodopsin conjugate, with added retinal, was refolded in a mixed detergent-lipid micelle and had an absorption maximum at 555 nm. The refolded conjugate was transferred into vesicles that pumped protons, upon illumination, as efficiently as did native BR. Modification of the PM with m-PEG did not alter the native structure or inhibit proton pumping, and therefore it is suggested that the glycol polymer is present as a moiety covalently linked to residues unnecessary for proton pumping and proper folding. The site of attachment of m-PEG was determined to be at either Lys 129 or Lys 159, with position Lys 129 the most probable site of attachment. The m-PEG-SC-BR33 could be stepwise refolded to the native conformation by the addition of trifluoroethanol to lower the dielectric constant, simulating the insertion of the BR into the phospholipid bilayer.
authors.affiliation: Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254-9110.
authors.forename: G
authors.initials: G
authors.lastname: Sirokmán
authors.forename: G D
authors.initials: GD
authors.lastname: Fasman
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D002942:Circular Dichroism; D008027:Light; D008081:Liposomes; D008954:Models, Biological; D008970:Molecular Weight; D011092:Polyethylene Glycols / Q000737:chemistry*; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017494:Proton Pumps / Q000528:radiation effects; D012172:Retinaldehyde; D012995:Solubility; D013053:Spectrophotometry; D013388:Succinimides / Q000737:chemistry*; D014270:Trifluoroethanol
chemical_list: D008081:Liposomes; D017494:Proton Pumps; D013388:Succinimides; D011092:Polyethylene Glycols; D001436:Bacteriorhodopsins; D014270:Trifluoroethanol; C069871:methoxypolyethylene glycol succinimidylsuccinate; D012172:Retinaldehyde