Disease relevance of SAG

• 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 [1] .
• Purified bovine retinal S-antigen was shown to produce a focal chorioretinitis, characterized by selective damage to the outer retina and, almost exclusively, a mononuclear cell infiltration of the choroid and retina [2].
• Autoimmune uveitis induced by molecular mimicry of peptides from rotavirus, bovine casein and retinal S-antigen [3].
• Using the enzyme-linked immunosorbent assay, the serum antibody titers to bovine retinal S-antigen and rod outer segments in pre- and post-argon laser treated diabetic retinopathy patients were compared to each other and to the titers of age- and sex-matched normal controls [4].

High impact information on SAG

• These processes were visualized immunocytochemically in the Djungarian hamster, Phodopus sungorus, with an antiserum against bovine retinal S-antigen and traced as far as the region of the posterior commissure and habenular nuclei [5] .
• Experiments using a bacterially expressed fusion protein show that the 48-kDa protein is capable of autophosphorylation on serine and tyrosine and phosphorylation of bovine beta-casein on serine, confirming that the 48-kDa protein itself has protein kinase activity [6].
• One of the early events during adhesion-induced signal transduction is the rapid inactivation of a flagellar protein kinase that phosphorylates a 48-kDa protein in the flagella [6].
• Chromogranin A (CGA) is a ubiquitous 48-kDa secretory protein present in adrenal medulla, anterior pituitary, central and peripheral nervous system, endocrine gut, thyroid, parathyroid, and endocrine pancreas [7].

Biological context of SAG

• The complete amino acid sequence of bovine S antigen (48-kDa protein) has been determined by cDNA and partial amino acid sequencing [8].
• Inactivation of photolyzed rhodopsin requires phosphorylation of the receptor and binding of the 48-kDa regulatory protein arrestin [9].
• Kinetics, binding constant, and activation energy of the 48-kDa protein-rhodopsin complex by extra-metarhodopsin II [10].
• If rhodopsin has been phosphorylated and regenerated prior to the protein binding experiment, the binding of 48-kDa protein depends on light but no longer on the presence of ATP [11].
• Excess GTP-binding protein thereby displaces 48-kDa protein from phosphorylated disks; this indicates competition between these two proteins for binding sites on illuminated phosphorylated rhodopsin molecules [11].

Anatomical context of SAG

• In contrast, PDEase activation or deactivation with unphosphorylated control membranes was not influenced by 48-kDa protein, even in the presence of ATP, provided rhodopsin kinase was absent [12].
• Phosphodiesterase activation by photoexcited rhodopsin is quenched when rhodopsin is phosphorylated and binds the intrinsic 48-kDa protein of rod outer segments [12].
• We have found that the 48-kDa protein (or S-antigen 48k) of the rod photoreceptor enhances the light-induced formation of the photoproduct metarhodopsin II (MII) from prephosphorylated rhodopsin [10].
• Light-induced binding of 48-kDa protein to photoreceptor membranes is highly enhanced by phosphorylation of rhodopsin [11].
• In vitro footprinting studies using a bovine mitochondrial extract and a series of deleted mtDNA templates identified a approximately 48-kDa protein which binds specifically to a single TAS element also protected within the mitochondrion [13].

Associations of SAG with chemical compounds

• These data show that although 48-kDa protein has ATPase activity, lack of regulation of this activity by the elements of visual transduction makes it unlikely for this activity to have a role in quenching the light activation of cyclic GMP cascade [14].
• Examination of native and reconstituted vesicles by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed two protein bands that were substantially enriched in the reconstituted system: one at 48 kDa and a diffuse band centered at 82 kDa [15].
• Purified bovine retinal S-antigen (50,000 m.w.) was treated with cyanogen bromide, producing seven major and several minor fragments [16].
• DNA polymerase delta was purified to apparent homogeneity by a four-column procedure including DEAE-Sephacel, phenyl-Sepharose, phosphocellulose, and hydroxylapatite, yielding two polypeptides of 125 and 48 kDa, respectively [17].
• In this study, we show, by pulse-chase labeling and glycosidase digestion, that LOX-1 is synthesized as a 40-kDa precursor protein with N-linked high mannose carbohydrate chains (pre-LOX-1), which is subsequently further glycosylated and processed into the 48-kDa mature form within 40 min [18].

Enzymatic interactions of SAG

• Our data suggest that 48-kDa protein binds to phosphorylated R* and thereby quenches its capacity to activate transducin and PDEase [12].

Other interactions of SAG

• Arrestin (also called S-antigen or 48-kDa protein) binds to photoexcited and phosphorylated rhodopsin and, thereby, blocks competitively the activation of transducin [19].
• Both lines are specific for S-antigen presented by histocompatible antigen-presenting cells, and also respond in vitro to several of the peptides produced by cyanogen bromide cleavage of bovine retinal S-antigen [20].
• Identification by amino acid sequencing revealed that the heat-sensitive group of lens proteins comprised glyceraldehyde-3-phosphate dehydrogenase (M(r) approximately 39 kDa), enolase (approximately 48 kDa), leucine aminopeptidase (approximately 52 kDa) and aldehyde dehydrogenase (approximately 53 kDa) [21].
• With the use of antisera against bovine retinal S-antigen and bovine opsin the authors demonstrate that in cerebellar medulloblastomas certain tumor cells display immunocytochemical properties characteristic of retinal photoreceptors and pinealocytes [22].
• Partial amino acid sequencing permitted the identification of these proteins as alpha-enolase (48 kDa), gamma-enolase (48 kDa), Glucose-6-phosphate-Isomerase (50 kDa) and aspartate-amino-transferase (46 kDa) [23].

Analytical, diagnostic and therapeutic context of SAG

• Sequence analysis of bovine retinal S-antigen. Relationships with alpha-transducin and G-proteins [24] .
• Proteins immunoprecipitated by this antibody from virus-infected cells ran at 36 and 43 kDa in reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and 43 and 48 kDa in nonreducing SDS-PAGE [25].
• Serum samples from 91 uveitis patients and controls were tested by the enzyme-linked immunosorbent assay technique to determine their relative antibody titers to bovine retinal S antigen and to a detergent extract of saline-washed bovine retina, or P antigen [26].
• Purification of retinal S-antigen to homogeneity by the criterion of gel electrophoresis silver staining [27].

References