Retinal Arrestin Research Articles

Polypeptide variants of beta-arrestin and arrestin3

Retinal arrestin (S-antigen) inactivates the phototransduction cascade by binding to light-activated phosphorylated rhodopsin and thereby "arresting" coupling to the G protein transducin. beta-Arrestin (beta arr), a ubiquitous arrestin homolog, acts analogously to desensitize the beta 2-adrenergic receptor by disrupting Gs receptor interaction. In an attempt to identify additional "arrestins" which might regulate the multitude of G protein-coupled receptors, we have isolated two bovine brain cDNAs which encode polypeptide variants of an arrestin homolog which we have designated arrestin3 (arr3). The open reading frames of these two cDNAs are identical except that the long form, arr3L, contains an 11-amino-acid insert between residues 361 and 362. Arr3 is more closely related to bovine beta arr (78% identity) than to bovine visual arrestin (56% identity). Polymerase chain reaction amplification of RNA and immunoblotting of lysates with an arr3-specific antibody suggest that the short form, arr3S, is the major form of arr3 in all bovine tissues and that it is most abundant in the spleen. Furthermore, polymerase chain reaction amplification of beta arr mRNA indicates that in several tissues (lung, liver, spleen, and pituitary), the major form of beta arr lacks 8 amino acids which are present in brain beta arr. Immunoblotting with an antibody which recognizes beta arr and arr3 with equal sensitivity demonstrates that beta arr (either the long or the short polypeptide) is the major arrestin in all (non-photoreceptor bearing) tissues examined. These observations suggest that in some tissues, as many as four arrestin homolog variants may play a role in the regulation of G protein-coupled receptors.

Cloning of a member of the arrestin family from a human thyroid cDNA library

We used the cDNA of human retinal arrestin as a probe to screen a human thyroid cDNA library. We isolated and plaque-purified one clone (hTHY-ARRX). The nucleotide sequence of the 1.8 kb cDNA insert had an open reading frame of 1227 bp coding for a protein of 409 amino acids. Northern blot analysis revealed a single transcript of 1.7 kb in human thyroid cells. There is significant homology between amino acid sequences of human thyroid arrestin and human retinal arrestin (63%) and bovine β-arrestin (74%), respectively. The hTHY-ARRX cDNA was stably transfected into Chinese hamster ovary cells already expressing a functional human thyrotropin (TSH) receptor. The cAMP response to TSH stimulation was unaltered in these cells, and homologous desensitization to TSH stimulation was not restored. It is not presently known whether hTHY-ARRX is human β-adrenergic arrestin or a new member of the arrestin family.

Desensitization of the isolated beta 2-adrenergic receptor by beta-adrenergic receptor kinase, cAMP-dependent protein kinase, and protein kinase C occurs via distinct molecular mechanisms.

Exposure of beta 2-adrenergic receptors (beta 2ARs) to agonists causes a rapid desensitization of the receptor-stimulated adenylyl cyclase response. Phosphorylation of the beta 2AR by several distinct kinases plays an important role in this desensitization phenomenon. In this study, we have utilized purified hamster lung beta 2AR and stimulatory guanine nucleotide binding regulatory protein (Gs), reconstituted in phospholipid vesicles, to investigate the molecular properties of this desensitization response. Purified hamster beta 2AR was phosphorylated by cAMP-dependent protein kinase (PKA), protein kinase C (PKC), or beta AR kinase (beta ARK), and receptor function was determined by measuring the beta 2AR-agonist-promoted Gs-associated GTPase activity. At physiological concentrations of Mg2+ (less than 1 mM), receptor phosphorylation inhibited coupling to Gs by 60% (PKA), 40% (PKC), and 30% (beta ARK). The desensitizing effect of phosphorylation was, however, greatly diminished when assays were performed at concentrations of Mg2+ sufficient to promote receptor-independent activation of Gs (greater than 5 mM). Addition of retinal arrestin, the light transduction component involved in the attenuation of rhodopsin function, did not enhance the uncoupling effect of beta ARK phosphorylation of beta 2AR when assayed in the presence of 0.3 mM free Mg2+. At concentrations of Mg2+ ranging between 0.5 and 5.0 mM, however, significant potentiation of beta ARK-mediated desensitization was observed upon arrestin addition. At a free Mg2+ concentration of 5 mM, arrestin did not potentiate the inhibition of receptor function observed on PKA or PKC phosphorylation. These results suggest that distinct pathways of desensitization exist for the receptor phosphorylated either by PKA or PKC or alternatively by beta ARK.

Physicochemical characterization of bovine retinal arrestin

The native conformation of bovine retinal arrestin has been characterized by a variety of spectroscopic methods. The purified protein gives rise to a near uv absorption band centered at 279 nm which results from the absorbance of its 14 tyrosine and one tryptophan residue. The extinction coefficient for this absorption band was determined to be 38.64 m m −1, cm −1 using the tyrosinate-tyrosine difference spectrum method; this extinction coefficient is ca. 17% lower than the previously reported value, and provides estimates of protein concentration which are in good agreement with estimates from the Bradford colorimetric assay. When native arrestin is purified to homogeneity, it displays a fluorescence spectrum which is dominated by tyrosine emission with no discernible contribution from tryptophan. Observation of the tyrosine-like fluorescence is dependent on the purity and structural integrity of the protein. Denaturation of arrestin by guanidine hydrochloride results in a diminution of tyrosine fluorescence and the concomitant appearance of a second fluorescence maximum at ca. 340 nm, presumably due to the single tryptophan residue. Thermal denaturation of arrestin leads to a conformation characterized by a broad fluorescence band centered at ca. 325 nm. Study of the arrestin fluorescence spectrum as a function of temperature indicates that the thermal denaturation is well modeled as a two-state transition with a transition midpoint of 60 °C. Temperature-dependent far uv circular dichroism studies indicate that changes in secondary structure occur coincident with the change in fluorescence. Studies of the temperature dependence of arrestin binding to light-adapted phosphorylated rhodopsin shows a strong correlation between the fluorescence spectral features of arrestin and its ability to bind rhodopsin. These data suggest that the relative intensities of tyrosine and tryptophan fluorescence are sensitive to the structural integrity of the native (i.e., rhodopsin binding) state of arrestin, and can thus serve as useful markers of conformational transitions of this protein. The lack of tryptophan fluorescence for native arrestin suggests an unusual environment for this residue. Possible mechanisms for this tryptophan fluorescence quenching are discussed.

Distribution of arrestin-like protein and β-subunit of GTP-binding proteins in quail choroid plexuses

Monoclonal antibodies (Mabs) directed against retinal arrestin (S-antigen) were used to detect and characterize this protein in choroid plexus (CP) of quails maintained during eight days, either under long-day photoperiods or in constant darkness. Immunocytochemistry and Western blotting confirmed the presence and the distribution of an arrestin-like protein in quail CP. Arrestin-like immunoreactivities in CP were compared with those obtained with Mabs to β 36-subunit of G proteins (Gβ), α-subunit of transducin and rhodopsin. Rhodopsin-like and transductin-like proteins could not be detected in choroidal cells, whereas intense positive reactions were observed with anti-Gβ and anti-arrestin Mabs. The strongest immunoreactivities were found in choroidal ependymocytes of the lateral and IIIrd ventricles. In CP epithelial cells lining the IVth ventricle, very weak or no immunoreactivity could be detected with Mabs to arrestin, while Mab against Gβ subunit always provided a positive reaction. In quails maintained in constant darkness, arrestin- and Gβ-immunoreactivities of CP epithelial cells displayed changes in cellular distribution and intensity (decrease or disappearance of the immunoreactions). The strong arrestin-like immunoreaction located in the apical region of ependymocytes suggests the preferential association of the protein with choroidal microvilli and a possible role in cerebrospinal fluid production assumed by CP cells.

Arrestin from nucleated red blood cells binds to bovine rhodopsin in a light-dependent manner

Using a panel of monoclonal antibodies, it has previously been demonstrated that the cytosol of nucleated red cells (trout and turkey) contains a protein similar to arrestin, a soluble protein found so far only in the photosensitive cells and which, by binding to photoexcited rhodopsin, inhibits the phototransduction process. The role of this arrestin-like protein in non-photosensitive cells is questionable. In this report we present evidence that partially purified red blood cell arrestin (RBC arrestin) behaves functionally like bovine retinal arrestin: it binds to phosphorylated bovine rhodopsin only when this receptor has been photoactivated. Thus RBC arrestin and bovine retinal arrestin are closely related both structurally and functionally. By analogy with the function of retinal arrestin, it is proposed that RBC arrestin is involved in desensitization of membrane transport proteins and/or adrenergic receptors.

Subspecies of arrestin from bovine retina

Arrestin (also named 48-kDa protein or S-antigen) binds to photoexcited and phosphorylated rhodopsin and thereby prevents activation of cGMP phosphodiesterase (EC 3.1.4.35) by transducin in retinal rods. We report here that retinal arrestin consists of several subspecies (isoelectric points between pH 5.5-6.2), which can be separated by FPLC anion-exchange chromatography and by FPLC chromatofocusing resulting in highly enriched individual subspecies. The entire heterogeneity pattern of arrestin is present in rod outer segments, independently of whether arrestin orginated from the outer or mostly from the inner segment of rod cells. The different subspecies show a similar binding behavior to photoexcited rhodopsin phosphorylated to various degrees and they quench the cGMP phosphodiesterase activity equally well. In the presence of rod outer segment membranes, arrestin is phosphorylated light-dependently by protein kinase C (0.2 mol phosphate/mol arrestin). This implies that the heterogeneity of arrestin is not primarily due to phosphorylation. Arrestin from different individuals exists as four isoelectric focusing patterns which occur with remarkably different frequencies in calf and cattle. The complexity of the IEF pattern does not increase with aging. Distinct subspecies of arrestin may reflect differences in their primary structure, or may result from differentially regulated post-translational modifications in individuals.

Immunological detection of arrestin, a phototransduction regulatory protein, in the cytosol of nucleated erythrocytes

Cytosolic extracts of trout and turkey erythrocytes were tested for their immunoreactivity with polyclonal and monoclonal antibodies to retinal arrestin (S-antigen), a cytosolic protein of photoreceptor cells involved in the desensitization of rhodopsin. After adsorption or immunoaffinity chromatography of the extracts, these antibodies specifically recognized a protein having a molecular weight similar to that of retinal arrestin. Because the G-protein-mediated transduction systems, such as visual and β-adrenergic systems, display a high degree of structural and functional homology, the presence of arrestin-like proteins in non-photosensitive cells suggests that these proteins are involved in the transduction of chemical signals, with a possible role in receptor desensitization.

Functional desensitization of the isolated beta-adrenergic receptor by the beta-adrenergic receptor kinase: potential role of an analog of the retinal protein arrestin (48-kDa protein).

The beta-adrenergic receptor kinase is an enzyme, possibly analogous to rhodopsin kinase, that multiply phosphorylates the beta-adrenergic receptor only when it is occupied by stimulatory agonists. Since this kinase may play an important role in mediating the process of homologous, or agonist-specific, desensitization, we investigated the functional consequences of receptor phosphorylation by the kinase and possible analogies with the mechanism of action of rhodopsin kinase. Pure hamster lung beta 2-adrenergic receptor, reconstituted in phospholipid vesicles, was assessed for its ability to mediate agonist-promoted stimulation of the GTPase activity of coreconstituted stimulatory guanine nucleotide-binding regulatory protein. When the receptor was phosphorylated by partially (approximately 350-fold) purified preparations of beta-adrenergic receptor kinase, as much as 80% inactivation of its functional activity was observed. However, the use of more highly purified enzyme preparations led to a dramatic decrease in the ability of phosphorylation to inactivate the receptor such that pure enzyme preparations (approximately 20,000-fold purified) caused only minimal (approximately 1off/- 7%) inactivation. Addition of pure retinal arrestin (48-kDa protein or S antigen), which is involved in enhancing the inactivating effect of rhodopsin phosphorylation by rhodopsin kinase, led to partial restoration of the functional effect of beta-adrenergic receptor kinase-promoted phosphorylation (41 +/- 3% inactivation). These results suggest the possibility that a protein analogous to retinal arrestin may exist in other tissues and function in concert with beta-adrenergic receptor kinase to regulate the activity of adenylate cyclase-coupled receptors.

Rapid affinity purification of retinal arrestin (48 kDa protein) via its light-dependent binding to phosphorylated rhodopsin

Arrestin (also named ‘48 kDa protein’ or ‘S-antigen’) is a soluble protein involved in controlling light-dependent cGMP phosphodiesterase activity in retinal rods, and is also known for its ability to induce autoimmune uveitis of the eye. We report a rapid and simple purification method based on the property of arrestin to bind specifically and reversibly to illuminated and phosphorylated rhodopsin [(1984) FEBS Lett. 176, 473–478]. This method does not require column chromatography and yields about 2–4 mg purified arrestin from 15 bovine retinas. Pure arrestin can be resolved by isoelectric focusing into at least 10 distinct bands, all of which stain with a monoclonal antibody specific for S-antigen.