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Structure and Function in Rhodopsin: the Fate of Opsin Formed Upon The Proc. Natl. Acad. Sci. USA Vol. 92, pp. 249-253, January 1995 Biochemistry Structure and function in rhodopsin: The fate of opsin formed upon the decay of light-activated metarhodopsin II in vitro (opsin unfolding/folding/denaturation/11-is-retinal/chromophore/regeneration) TAKESHI SAKAMOTO* AND H. GOBIND KHORANA Departments of Biology and Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 Contributed by H. Gobind Khorana, September 8, 1994 ABSTRACT We report that the light-activated bovine parallels the rate of decay of the Meta II intermediate. Thus, the metarhodopsin II, upon decay, first forms opsin in the cor- opsin formed upon decay of this intermediate regains the con- rectly folded form. The latter binds ll1-is-retinal and regen- formation ofthe native ground-state opsin. However, while being erates the native rhodopsin chromophore. However, when the kept in the dark in DM, the opsin converts to one or more opsin formed upon metarhodopsin II decay is kept in 0.1% non-11-cis-retinal-binding forms in a time-dependent manner. dodecyl maltoside, it converts in a time-dependent manner to Upon subsequent addition of 11-cis-retinal, a slow regeneration a form(s) that does not bind ll-cis-retinal. On subsequent of the native rhodopsin chromophore is observed. Usually, three addition of ll-cis-retinal, slow reversal of the non-retinal- rate components are observed for this process: the first, a rapid binding forms to the correctly folded retinal-binding form has one (t1l2 = 15-60 sec) ascribed to the surviving correctly folded been demonstrated. We have studied the influence, on the opsin, the second with a tly2 of some minutes, and the third with above interconversions, of pH, phospholipids (rod outer seg- a t1l2 of 6-10 hr. We document these findings and present ment and soybean), dithiothreitol, and a mixture of reduced additional observations on the influence of different conditions and oxidized glutathione. Chromophore regeneration in the on the reversal to the correctly folded opsin. The system appears presence of 11-cis-retinal was highest at pH 6.0-6.3. The promising for further studies of unfolding and refolding of the addition of dithiothreitol just before bleaching gave back only photoreceptor. a small amount (7%) of rhodopsin on the subsequent addition of ll-cis-retinal, whereas the slow phase(s) of chromophore MATERIALS AND METHODS formation was completely abolished. The presence of a mix- ture of reduced and oxidized glutathione did not significantly Materials. Bovine retinae were purchased from J. A. Law- affect the results. Addition of phospholipids, either from son Co. (Lincoln, NE); DM was from Anatrace (Maumee, soybean or rod outer segment, prior to bleaching stabilized the OH). 11-cis-Retinal was a gift of P. Sorter (Hoffmann-La initially formed opsin, resulting in much higher chromophore Roche) and R. Crouch (Medical University of South Carolina regeneration. However, addition of the phospholipids after and the National Eye Institute). conversion of the opsin to non-retinal-binding form(s) ar- Preparation of Delipidated Rod Outer Segment (ROS) rested the subsequent reversal of the opsin to the retinal- Rhodopsin. ROS membranes were prepared under dim red binding form. light (>650 nm) from frozen bovine retinae according to Wilden and Kuhn (7). Urea-washed ROS membranes were Unfolding and refolding of the seven-helix integral membrane prepared by the method of Shichi and Somers (8). The protein bacteriorhodopsin was studied in the early 1980s (1). membranes were suspended in 10% (wt/vol) sucrose contain- Efficient refolding from a completely denatured state was ing 5 mM Tris-HCl (pH 7.5), quickly frozen in liquid N2, and demonstrated for this protein (2-4). To date, subsequent stored at -70°C. ROS rhodopsin was prepared from the investigations to refold the dim-light photoreceptor bovine urea-washed ROS membranes by solubilization in 1% DM (9) rhodopsin, which contains the specialized three domains, from and was further purified (delipidated) by affinity chromatog- unfolded states have, however, been uniformly unsuccessful. raphy on Con A-Sepharose (Pharmacia) (9, 10). The Sepha- These attempts constituted a wide variety of conditions in- rose (250 ,ul) was washed six times by using, each time, 12 ml cluding the ideas developed for refolding of bacteriorhodopsin of 100 mM sodium phosphate buffer (pH 6.3) containing 150 (A. Kronis and H.G.K., unpublished work). We now report mM NaCl, 1 mM EDTA, and 0.1% DM, which is hereafter on the development of a partially reversible denaturation- denoted buffer A. The rhodopsin that bound to the Sepharose renaturation for this sensory protein. Rhodopsin upon was eluted with 0.3 M a-methylmannoside in buffer A (the system column was washed three times with 300 ,ul of the eluant each light activation undergoes 11-cis -- all-trans isomerization of the retinal chromophore; the metarhodopsin II (Meta II) time for a 250-,ul column of Sepharose). Rhodopsin was stored intermediate formed, after execution of its signal transduction in the dark at 4°C. The preparations showed UV/visible function, discards the bound molecule of all-trans-retinal, and absorbance ratios (A28o/A5oo) of 1.7-1.8. the resulting opsin is believed to recycle by re-forming the Soybean and ROS Phospholipids. Soybean phospholipids functional chromophore by capturing a new molecule of (asolectin) were purified by the method of Kagawa and Racker the nature and (11). ROS phospholipids were prepared according to Wiegand 11-cis-retinal (5). We have now investigated and Anderson (12) except that prior to lipid extraction ROS behavior of the opsin formed from light-activated Meta II in of in vitro systems, particularly in the detergent n-dodecyl P3-D- membranes were bleached for 20 min at 4°C in the presence maltoside (DM).t We find that in the presence of l1-cis-retinal, 20 mM hydroxylamine with a 300-W projector lamp using a added immediately after illumination of rhodopsin, the native rhodopsin chromophore regenerates and the rate of its formation Abbreviations: Meta II, metarhodopsin II; DM, dodecyl maltoside; DTT, dithiothreitol; ROS, rod outer segment. *On leave from Advanced Research Laboratory, Hitachi, Hatoyama, The publication costs of this article were defrayed in part by page charge Saitama 350-03, Japan. payment. This article must therefore be hereby marked "advertisement" in tThis is paper 10 in the series "Structure and Function in Rhodopsin." accordance with 18 U.S.C. §1734 solely to indicate this fact. Paper 9 is ref. 6. 249 Downloaded by guest on October 1, 2021 250 Biochemistry: Sakamoto and Khorana Proc NatZ Acad Sci USA 92 (1995) long-pass filter with a cutoff at 495 nm (Melles Griot, Irvine, Time-Dependent Conversion of the Opsin Initially Formed CA). Phosphorous was determined by the method of Fiske and upon Meta II Decay to Non-Retinal-Binding Species. Rho- SubbaRow as modified by Dittmer and Wells (13). dopsin was bleached at pH 6.3 by the standard protocol. The UV/Visible Spectroscopy. All UV/visible spectra were bleached sample was divided into aliquots, and the latter were measured with a Perkin-Elmer X6 or A7 UV/visible spectro- kept in the dark for varying lengths of time (1-24 hr). To each photometer, equipped with water-jacketed cuvette holders sample, 11-cis-retinal (1.5 mole equivalents) was added. Chro- connected to a circulating water bath (model RTE 5DD; mophore (A500) formation at room temperature was followed Neslab Instruments, Portsmouth, NH). Matched quartz cu- in the dark for up to 1 week. The results are shown in Fig. 2. vettes (1-cm light path, 0.2-cm-wide black side walls and base) The first striking observation was that the ability to regenerate were purchased from Hellma (Forest Hills, NY) or Uvonic the A500 chromophore rapidly upon addition of 11-cis-retinal Instruments (Plainview, NY). The average concentration of decreases with a longer incubation in the dark after bleaching. rhodopsin samples in buffer A was 0.8 ,uM rhodopsin (addi- Further, the extent of total regeneration also decreased with tions were as indicated in the legends to the figures). They were longer incubation before retinal addition. Thus, after 1 hr in illuminated in the quartz cuvettes with a 150-W fiber optic light the dark (Fig. 2, curve 1) without 11-cis-retinal, the extent of (Fiber Lite A-200; Dolan-Jenner, Woburn, MA) equipped chromophore regeneration was 78%, whereas after a 24-hr with a >495-nm long-pass filter. incubation (Fig. 2, curve 4), the chromophore regeneration Rate of Decay of Meta II. A rhodopsin sample prepared as was only 35%. Nonlinear curve fitting (SIGMAPLOT; Jandel, above was illuminated for 15 sec, long enough to completely Corte Madera, CA) showed three exponents in chromophore convert the initial 500-nm-absorbing species to the Meta TI regeneration: the first with a ti/2 of 15-60 sec, the second with form. Aliquots of the irradiated sample were acidified at a ti/2 of 20-30 min, and the third (the slowest) with a til2 of 6-10 different time intervals to pH 1.9 with 2 M H2SO4, and the hr. These exponents made different contributions to the total absorbance at 440 nm due to the remaining protonated Schiff chromophore regeneration. The contribution of the first (fast) base was recorded (14). phase, ascribed to the chromophore regeneration from the Hydroxylamine Treatment of500-nm-Absorbing Rhodopsin surviving correctly folded opsin, decreased (curves 1-4, Fig. 2) Regenerated with 11-cis-Retinal After Decay of Meta II. while the contribution from the third (slow) phase became Hydroxylamine hydrochloride (2 M, pH 7.0) was added in the more prominent.
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