Comparison of bacterial and animal rhodopsins by hydrogen exchange studies

Abstract

RHODOPSIN is deeply embedded in a hydrocarbon membrane, yet hydrogen exchange (HX) studies show that about 70% of its peptide hydrogens are freely exposed to solvent water¹. This contrasts with other membrane-bound proteins and even aqueous proteins which generally involve about 70% of their peptide hydrogens in internal H-bonding. It has been suggested that the large fraction of disordered polypeptide chain indicated for rhodopsin resides in and acts to stabilise a channel of water penetrating into the membrane, and further that this channel must be quite large, perhaps 12 Å or more in diameter¹. Unwin and Henderson² described an electron diffraction model for the purple membrane protein of Halobacterium halobium (bacteriorhodopsin) which compares interestingly with the suggested structure of animal rhodopsin; membrane-embedded bacteriorhodopsin is arranged in a three-molecule circle surrounding an open space 20 Å in diameter. This space, however, is filled not with water but with lipid, and the protein walls of this blind channel are formed of a double rank of α helices which account for most of the bacteriorhodopsin polypeptide chain, so that bacteriorhodopsin, unlike animal rhodopsin, must be as extensively H-bonded as is the usual protein. We report here a HX study of bacteriorhodopsin. In agreement with the electron diffraction model, our HX results show that this protein, unlike animal rhodopsin, has about 75% of its peptides internally H-bonded. These and previous results, however, suggest a fundamental structural similarity between the two rhodopsins.