Site of attachment of retinal in bacteriorhodopsin.

Abstract

The identity of the lysine residue in bacteriorhodopsin to which the chromophore, retinal, is attached as a Schiff's base has been reinvestigated. Retinal is now shown to be linked to lysine-216 and not lysine-41, as had been concluded previously. The retinal in purple membrane was replaced by [15-H]retinal, the Schiff's base linkage was reduced with NaBH4 while the sample was illuminated, and the resulting [retinyl-3H]bacterio-opsin was cleaved with CNBr. The radioactivity was present exclusively in the COOH-terminal pestide (amino acids 210-248). Sequence analysis showed that the [ H]retinal was attached to lysine-216.The same site was labeled when purple membrane was reduced with NaBH4 in the light at pH >10 or at pH 8 and when membranes modified with ether or solubilized with hexadecyltrimethylammonium bromide were reduced in the dark. Our finding places the Schiff's base linkage close to the midpoint ofthe putative membrane-spanning ae-helix that is directly connected to the COOH terminus of bacteriorhodopsin.Bacteriorhodopsin, the only protein in the purple membrane (PM) of Halobacterium halobium, catalyzes the light-driven vectorial translocation of protons and thus generates a transmembrane electrochemical gradient (1). The protein consists of a single polypeptide chain of 248 amino acids, whose amino acid sequence is now known (2, 3). A three-dimensional model of bacteriorhodopsin has been developed (4) that is compatible with the amino acid sequence, diffraction data (5, 6), and the susceptibility ofcertain regions ofthe polypeptide to proteolysis (7,8): the polypeptide is believed to traverse the membrane seven times in the form of a-helical rods.For further structural and mechanistic studies, it is important to know the site ofattachment ofretinal to the polypeptide. The chromophore is attached as a Schiff's base to the e-amino group of a lysine residue (9, 10), previously concluded to be lysine-41 (2, 3, 11) based on the results of Bridgen and Walker (12). We have reinvestigated the site of attachment; we find that retinal is attached to lysine-216. We compare our finding with the result of a recent study in which the site of attachment of retinal to bovine rhodopsin was determined (13 [Retinal-3H]PM (30 1d; 13.8 mg ofprotein ml-') or unlabeled PM at the same concentration and NaBH4 (22 mg ml-l) or NaB3H4 (25 mCi; 434 mCi mmol-1) in 50 mM Na2COJHCl, pH 10.0 (100 ul) were mixed at 00C and illuminated with a Kodak model 800 slide projector equipped with a Schott OG530 filter. The mixture was bleached within 10-15 min, and the reaction was then quenched by transferring the membranes to 100 mM sodium phosphate, pH 6.5 (1.3 ml). After 1 hr at 00C and 1 hr at 250C, the product was dialyzed against the same buffer for 1 day at 250C and then further washed by dialysis or by repeated centrifugation and suspension. Unlabeled reduced PM was produced by using the same procedure on a larger scale. An illuminated sample of [retinal-3H]PM (10 1.l; 13.8 mg ml-') was also completely reduced at pH 8 and 0C i...

In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors

“…This concept has its origins in physical studies on bacteriorhodopsin. The all-trans-retinal chromophore of bacteriorhodopsin is attached via a Schiff base to Lys216, located in the seventh transmembrane helix of the protein (Bayley et al, 1981;Mullen et al, 1981), and is contained within a pocket formed by the transmembrane helices (Henderson et al, 1990;see Fig. 1).…”

1H‐15N‐NMR studies of bacteriorhodopsin Halobacterium halobium

“…al., 1979). The retinal moiety is covalently bound via a Shiff base to the &-amino group of Lys216 (Bayley et al, 1981).…”

Retinal migration during dark reduction of bacteriorhodopsin

“…This peak is normally due to fragment CNBr-1 (19). Little, if any, retinyl absorbance comigrates with the only tryptophan-containing fragment, CNBr-2 (amino acids [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], identified by the characteristic shoulder near 290 nm (Fig. 2).…”