The rate of rhodopsin phosphorylation in isolated rentinas of frog and cattle

Phosphorylation of rod outer segment proteins modulates phosphatidylethanolamine N-methyltransferase and phospholipase A2 activities in photoreceptor membranes

The activities of enzymes involved in lipid metabolism--phospholipase A2 (PLA2) and phosphatidylethanolamine N-methyltransferase (PE N-MTase)--were found to be differently affected by pre-incubation of rod outer segments (ROS) under protein phosphorylating or dephosphorylating conditions. Exposure to cAMP-dependent protein kinase (PKA), under dark or light conditions, produced a significant increase in PE N-MTase activity, whereas PLA2 activity decreased. Under standard protein kinase C (PKC) phosphorylating conditions in light, PE N-MTase activity was stimulated and PLA2 activity was not affected. When the assays were performed in the dark, both enzymatic activities were unaffected when compared to the corresponding controls. Incubation of ROS membranes in light in the presence of PKC activators phorbol 12,13-dibutyrate (PDBu) and dioctanoylglycerol (DOG) resulted in the same pattern of changes in enzyme activities as described for standard PKC phosphorylating condition. Pre-incubation of membranes with the PKC inhibitor H-7 reduced the stimulation of PDBu on PE N-MTase activity, and had no effect on PLA2 activity in ROS membranes incubated with the phorbol ester. Pre-treatment of isolated ROS with alkaline phosphatase resulted in decreased PE N-MTase activity and produced a significant stimulation of PLA2 activity under dark as well as under light conditions when compared to the corresponding controls. These findings suggest that ROS protein phosphorylation and dephosphorylation modulates PE N-MTase and PLA2 activities in isolated ROS, and that these activities are independently and specifically modulated by particular kinases. Furthermore, dephosphorylation of ROS proteins has the opposite effect to that produced by protein phosphorylation on the enzymes studied.

Regulation of the phosphorylation state of rhodopsin by dopamine

G protein-coupled receptors (GPCRs) are regulated by kinases and phosphatases that control their phosphorylation state. Here, the possibility that the state of GPCR phosphorylation could be affected by paracrine input was explored. We show that dopamine increased the rate of dephosphorylation of rhodopsin, the light receptor, in intact frog retinas. Further, we found that rod outer segments from dopamine-treated retinas contained increased rhodopsin phosphatase activity, indicating that this effect of dopamine on rhodopsin was mediated by stimulation of rhodopsin phosphatase. Dopamine is a ubiquitous neuromodulator and, in the retina, is released from the inner cell layers. Thus, our results identify a pathway for feedback regulation of rhodopsin from the inner retina and illustrate the involvement of dopamine in paracrine regulation of the sensitivity of a GPCR.

Phosphorylation of non-bleached rhodopsin in intact retinas and living frogs

The photoresponse in retinal photoreceptors begins when a molecule of rhodopsin is excited by a photon of light. Photoexcited rhodopsin activates an enzymatic cascade including the G-protein transducin and cyclic GMP phosphodiesterase. As a result, cytoplasmic cyclic GMP concentration is decreased and the photoresponse is initiated. This process is terminated when rhodopsin is phosphorylated by rhodopsin kinase and subsequently blocked by a protein called arrestin. It has been noted by several investigators that light can cause phosphorylation of not only photoexcited but also non-excited rhodopsin in rod photoreceptors. A goal of this study was to determine how much non-bleached rhodopsin is phosphorylated. To determine how the structural integrity of the photoreceptor influences the extent of non-bleached rhodopsin phosphorylation, we studied the reaction in electropermeabilized rod outer segments, in rod outer segments still attached to isolated retinas and in living frogs. In the first two preparations, we found that the maximum extent of non-bleached rhodopsin phosphorylation was approximately 1% of the total rhodopsin pool. In living frogs, the maximal amount of non-bleached rhodopsin phosphorylation was approximately 2% of the total rhodopsin pool and occurred after prolonged illumination by the relatively dim light intensity of 20 lux. These data appear to exclude models for light adaptation that postulate high levels of phosphorylation of non-bleached rhodopsin in rod photoreceptors.

Dim background light and Cerenkov radiation from 32P block reversal of rhodopsin phosphorylation in intact frog retinal rods

The phosphorylation of photoexcited rhodopsin (Rho*) is thought to inactivate this receptor by inhibiting its interaction with the GTP-binding protein transducin (Gt). Here we report that the time course of phosphorylation-dephosphorylation after bright illumination of intact rod outer and inner segments (ROS-RIS) incubated in 33Pi can be altered if the ROS-RIS are first exposed to levels of dim illumination that cause light adaptation in these ROS-RIS. The dephosphorylation of greater than 10(7) phosphorylated rhodopsin molecules/ROS following a bright flash can be blocked by prior dim continuous illumination (generating 10(3) Rho*/ROS/s) that cumulatively bleaches approximately 10(5) rhodopsin molecules/ROS. The phenomenon has not been previously noted because these low levels of light are emitted as a result of Cerenkov radiation from the 32P isotope that is usually employed to monitor rhodopsin phosphorylation. The inhibition of rhodopsin dephosphorylation by dim conditioning illumination is observed in intact ROS-RIS but is lost when ROS-RIS are electropermeabilized or fragmented.

C-terminal peptides of rhodopsin. Determination of the optimum sequence for recognition of retinal transducin

In vertebrate retinal rod outer segments, transducin, a guanine-nucleotide-binding protein, mediates signal coupling between rhodopsin and cyclic GMP phosphodiesterase. Whereas the T alpha subunit (39 kDa) of transducin binds guanine nucleotides and is the activator of the phosphodiesterase, the T beta gamma subunits (35 and 10 kDa) may function to physically link T alpha with photolysed rhodopsin. We have previously reported that a site of binding of transducin is on the C-terminus of bovine rhodopsin. By using competition with synthetic peptides, the recognition region was localized to bovine opsin amino acid residues 317-339. Further studies are detailed which determine the boundaries of this binding site on rhodopsin, as well as some of the critical amino acids needed for transducin binding. These results suggest that the serine and threonine residues in the rhodopsin C-terminal peptides Rhod-1 and Rhod-3 are critical for reconstitution of transducin GTPase activity.

Regulation of retinal transducin by C-terminal peptides of rhodopsin

Transducin is a multi-subunit guanine-nucleotide-binding protein that mediates signal coupling between rhodopsin and cyclic GMP phosphodiesterase in retinal rod outer segments. Whereas the T alpha subunit of transducin binds guanine nucleotides and is the activator of the phosphodiesterase, the T beta gamma subunit may function to link physically T alpha with photolysed rhodopsin. In order to determine the binding sites of rhodopsin to transducin, we have synthesized eight peptides (Rhod-1 etc.) that correspond to the C-terminal regions of rhodopsin and to several external and one internal loop region. These peptides were tested for their inhibition of restored GTPase activity of purified transducin reconstituted into depleted rod-outer-segment disc membranes. A marked inhibition of GTPase activity was observed when transducin was pre-incubated with peptides Rhod-1, Rhod-2 and Rhod-3. These peptides correspond to opsin amino acid residues 332-339, 324-331 and 317-321 respectively. Peptides corresponding to the three external loop regions or to the C-terminal residues 341-348 did not inhibit reconsituted GTPase activity. Likewise, Rhod-8, a peptide corresponding to an internal loop region of rhodopsin, did not inhibit GTPase activity. These findings support the concept that these specific regions of the C-terminus of rhodopsin serve as recognition sites for transducin.

Rhodopsin phosphorylation inhibited by adenosine in frog rods: lack of effects on excitation

The rod photocurrent was studied by recording the transretinal voltage from the aspartate-treated isolated frog retina before and after perfusion with 2 mM adenosine, which inhibited 60-80% of the light-induced rhodopsin phosphorylation. Adenosine did not affect the time courses of the flash photoresponses or the OFF responses after a steady light. The introduction of adenosine while the retina was illuminated by a steady background did not enhance the effect of light. Instead, the opposite change, due to PDE inhibition, was observed. The results indicate that rhodopsin phosphorylation does not determine the time course of the decay of excitation.

Activation of rod outer segment phosphodiesterase by enzymatically altered rhodopsin: a regulatory role for the carboxyl terminus of rhodopsin

Soluble enzymes, extracted from bovine retinal rod outer segments (ROS), were recombined with native ROS discs and discs which had been modified either by protease treatment or phosphorylation with rhodopsin kinase. The effect of these modifications on rhodopsin's ability to light-activate the ROS phosphodiesterase was determined. Trypsin, short-term thermolysin, and papain-digested discs were more effective in activating the phosphodiesterase than were undigested discs, whereas phosphorylated discs showed reduced ability to activate the phosphodiesterase. When a non-hydrolyzable analogue was employed in place of GTP in the assay, the same differences in the activation of phosphodiesterase as described above were observed between control discs and discs which were digested with thermolysin or phosphorylated. The proteolysis treatments remove various segments of amino acids from the carboxyl terminus of rhodopsin. In addition, at least seven phosphorylation sites are located in the terminal 15 amino acid residues of the carboxyl terminus of rhodopsin. Hence, it would appear from these studies that modifications of rhodopsin which affect the carboxyl terminus result in marked changes in the level of light-activatable phosphodiesterase activity, strongly suggesting a regulatory involvement in the light-activation process for this portion of rhodopsin.

Cyclic GMP stimulation of a light-activated ATPase in rod outer segments

Rod outer segments (ROSs) of vertebrate photoreceptor cells have been reported to contain several enzyme systems including a dark, Ca2+-stimulated ATPase, a rhodopsin kinase, a phosphodiesterase and a GTPase, all of which are light-stimulated. Recently, Thacher has found a light-stimulated Mg2+-ATPase in frog ROSs while our own laboratory has identified a dark, Ca2+-inhibited Mg2+-ATPase in bovine ROSs. Here we extend our observations on the Mg2+-ATPase and demonstrate that flash illumination following the dark ATPase process stimulated ATPase activity at a rate considerably faster than the dark process. In addition, we find that both the dark and light stimulated ATPase activities are markedly enhanced by cyclic GMP and inhibited by Ca2+.

Metabolism in the cytosol of intact isolated cattle rod outer segments as indicator for cytosolic calcium and magnesium ions

The metabolism of the chromophore of rhodopsin in the cytosol compartment of isolated intact cattle rod outer segments was used as an indicator for changes of the cytosolic Mg2+ and Ca2+ concentration upon changes of the external Mg2+ and Ca2+ concentration. The reduction of retinal to retinol upon photolysis of rhodopsin in situ in intact rod outer segments was critically dependent on the availability of cytosolic Mg2+. The latter is necessary as chelator of endogenous adenosine 5'-triphosphate (ATP). Lowering the cytosolic Ca2+ concentration beneath 10(-7) M resulted in an inhibition of the rate of retinol formation. This is presumably due to a light-activated process, which competes with retinol formation for the supply of high-energy phosphate from a common pool. These results led to the following conclusions. Changes of the external Mg2+ concentration are only followed by substantial changes of the cytosolic Mg2+ concentration when the ionophore A23187 is present. Changes of the external Ca2+ concentration are followed by parallel changes of the cytosolic Ca2+ concentration either when external Na+ is present or in the presence of A23187. Li+ and K+ could not substitute for Na+ in the former case, but K+ diminished the effectivity of Na+ at low Na+ concentrations and enhanced it at high Na+ concentrations. It is concluded that the control of cytosolic Ca2+ concentration in isolated intact rod outer segments is predominantly provided for by Na-Ca exchange, i.e., by coupled fluxes.

Phosphorylation of rhodopsin as a possible mechanism of adaptation

Light-induced phosphorylation of rhodopsin has been extensively studied by a number of investigators from a biochemical point of view. However, little is known about the physiological function of this reaction. The slow rates measured for phosphorylation and dephosphorylation suggest that it may be involved in visual adaptation rather than in excitation. This paper presents biochemical data obtained from phosphorylation experiments in isolated photoreceptor membranes as well as in the physiological system of whole retinas and living animals. An attempt is made to compare the phosphorylation reaction with visual adaptation hypotheses taken from the electrophysiological literature. Finally, effects of cyclic nucleotide metabolism on the sensitivity of photoreceptors are presented and discussed.

The amino- and carboxyl-terminal sequence of bovine rhodopsin

The amino terminus of bovine rhodopsin is blocked and has the sequence x-Met-Asn(CHO)-Gly-Thr-Glu-Gly-Pro-Asn-Phe-Tyr-Val-Pro-Phe-Ser-Asn(CHO)-Lys-Thr-Gly-Val-Val-Arg, where CHO represents sites of carbohydrate attachment. The carboxyl-terminal sequence of rhodopsin is Val-Ser-Lys-Thr-Glu-Thr-Ser-Gln-Val-Ala-Pro-Ala. Upon short-term digestion of rod outer segment (ROS) membranes with thermolysin, opsin (similar to 35,000 daltons) is converted to a membrane-bound fragment O' (similar to 30,500 daltons) and 2 peptides containing 12 amino acids are released from the carboxyl terminus of rhodopsin into the supernatant. Upon long-term digestion of ROS with thermolysin, opsin and O' are replaced by the membrane-bound fragments F1 (similar to 25,000 daltons), and F2 (similar 9,500 daltons). When 32P-ROS are digested, F2 carries the 32P. Both O' and F1 contain the amino-terminal glycopeptide.