Disease relevance of Sag

• The open reading frame contained within the long terminal repeat (LTR) of mouse mammary tumor virus encodes Naf, a negative regulator of transcription, as well as a superantigen activity, Sag, which causes the deletion of specific classes of T cells [1].
• Expression of Sag proteins from endogenous MMTV proviruses carried in the mouse germ line usually results in the deletion of self-reactive T cells during negative selection in the thymus and the elimination of T cells required for infection by specific milk-borne MMTVs [2].
• The 48-kDa protein is constitutively myristoylated in murine macrophages that have been activated in vivo by intraperitoneal injection of Corynebacterium parvum, suggesting that it may be an early intermediate in the activation of macrophages [3].
• Identification of proteins involved in intracellular copper metabolism. Low levels of a approximately 48-kDa copper-binding protein in the brindled mouse model of Menkes disease [4].
• Clostridium septicum lethal (alpha-toxin) was purified and found to be a basic protein (pI 8.4) of approximately 48 kDa that is both lethal and hemolytic [5].

High impact information on Sag

• Here we identify a 48 kDa type I transmembrane glycoprotein encoded by the MCMV early gene m06, which tightly binds to properly folded beta2-microglobulin (beta2m)-associated MHC class I molecules in the endoplasmic reticulum (ER) [6].
• IGTP is the prototype for a family of IFN-gamma-regulated genes that encode 48-kDa GTP-binding proteins that localize to the endoplasmic reticulum [7].
• During the induction of long-term potentiation (LTP) in hippocampal slices adenosine triphosphate (ATP) is secreted into the synaptic cleft, and a 48 kDa/50 kDa protein duplex becomes phosphorylated by extracellular ATP [8].
• Neither IFN-alpha nor IFN-beta has any effect on IFN-gamma-induced myristoylation of the 48-kDa protein [3].
• Interferon gamma induces the myristoylation of a 48-kDa protein in macrophages [3].

Biological context of Sag

• We have used a mouse cDNA clone for the S-antigen to map the corresponding gene, Sag, to mouse chromosome 1 near Idh-1 [9].
• The genes discovered by this approach are the full-length mouse homologue of guanylate cyclase 2F (GUCY2F) and a carboxy-truncated splice variant of retinal S-antigen (SAG), known as regulators of the visual phototransduction G-protein-coupled receptor-mediated signaling pathway [10].
• Little is known about the structure, intracellular trafficking, or nature of Sag association with major histocompatibility (MHC) class II products [11].
• In order to gain a better understanding of Sag structure-function relationships, we extensively mutagenized this type II glycoprotein using two different approaches: transposon-mediated random in-frame insertion mutagenesis and site-directed mutagenesis targeting clusters of charged residues [11].
• Surprisingly, similar effects were observed with Sag mutants with substitutions at pairs of charged residues; only 2 of 6 mutants trafficked to the plasma membrane and stimulated T cells, 1 with a temperature-sensitive phenotype [11].

Anatomical context of Sag

• S-antigen (48-kDa protein) is a soluble protein of the retina and the pineal gland that is believed to play an important role in the visual process [9].
• Since the Sag encoded by LA virus strongly stimulated cognate T cells in vivo, selection for recombinant virus with the Mtv-7 sag most likely occurred because the increased T-cell proliferation resulted in greater lymphoid and mammary gland cell infection [12].
• The 48-kDa protein immunoreactivity, which is observed in the whole photoreceptor layer both in rd and control retinas throughout development, is the only one of all immunoreactivities analysed that remains at 2 months of age in the rd retina and is probably localized in cones [13].
• Mouse mammary tumor virus (MMTV) encodes a superantigen (Sag) that is expressed at the surface of antigen-presenting cells in conjunction with major histocompatibility complex (MHC) type II molecules [2].
• Immunoblot analysis confirmed that this antibody recognized the original immunizing protein as well as a 44- to 48-kDa protein from several raf-transformed cell lines [14].

Associations of Sag with chemical compounds

• The dialyzed FA-1 contained a major 23-kDa and a minor 48-kDa band when separated on both sucrose and cesium chloride gradients [15].
• Lysine binding fragments of plasmin, isolated from macrophage-conditioned media utilizing affinity chromatography, appeared as a major (48 kDa) and two minor bands (42 and 50 kDa) in SDS-polyacrylamide gel electrophoresis and were immunoreactive with anti-kringle 1-3 IgG [16].
• Calpain, a Ca(2+)-dependent cysteine protease, in vitro converts calcineurin (CaN) to constitutively active forms of 45 kDa and 48 kDa by cleaving the autoinhibitory domain of the 60 kDa subunit [17].
• RESULTS: Lumican was present in the mouse sclera as an approximately 48-kDa core protein containing short glycosaminoglycan side chains consisting of moderate- to low-sulfated keratan sulfate [18].
• Retinal S-antigen was demonstrated in the WERI-Rb1 and to a lesser extent the Y-79 tissue cultured retinoblastoma cell lines as well as an ethanol-fixed, paraffin-embedded retinoblastoma by an indirect immunoperoxidase technique using monoclonal antibody MAbA9-C6, and by flow cytometric analysis (FCM) using MAbA9-C6 and MAbA1-G5 [19].

Physical interactions of Sag

• The 97 kDa receptor in cytosol from wild type cells was digested by chymotrypsin to a 40 kDa steroid-binding receptor fragment but the 48 kDa receptor in cytosol from nti mutants was resistant to digestion by chymotrypsin [20].

Enzymatic interactions of Sag

• Caspase-3 also cleaved PDE10A2 (molecular mass of 95 kDa) to a single 48-kDa fragment [21].

Other interactions of Sag

• Tp-1 mapped 0.8 +/- 0.8 cM proximal and Sag 12.8 +/- 1.7 cM distal to (Vil,Bcg) [22].
• Since the rd gene is located on mouse chromosome 5, our results suggest that Sag is not the site of the rd mutation [9].
• G beta and 48-kDa protein mRNAs are already detectable at birth, opsin mRNA appears by postnatal day 5 (P5), G gamma mRNA at P6 and G alpha mRNA by P8 [13].
• We find that the levels of each mRNA analysed appear to cycle in the +/+ adult retina, with the greatest amount of opsin and the three subunits of G-protein mRNAs occurring just before light onset and the greatest amount of 48-kDa protein mRNA occurring just before lights off [13].
• Western blot analysis revealed a 48 kDa V1a and a 55 kDa OXT receptor immunoreactive band that was expressed in tissue obtained from L1-L6 sections of spinal cord [23].

Analytical, diagnostic and therapeutic context of Sag

• Immunocytochemistry indicates that the 48-kDa protein is present at birth, G gamma and opsin are detectable at P4 and G alpha at P7 [13].
• To investigate the kinetics of Sag-reactive T-cell deletion, backcross mice that contain single or multiple MMTVs were screened by a novel PCR assay designed to distinguish among highly related MMTV strains [2].
• An anti-peptide antibody raised against the carboxyl-terminal 11 amino acids of mouse GPT, immunoadsorbed GPT activity and recognized a protein of expected size (approximately 48 kDa) on Western blots [24].
• A soluble, 391-amino acid single-chain H-2Kd (SC-Kd) molecule of 48 kDa was synthesized and glycosylated in insect cells and could be purified in the absence of detergents by affinity chromatography using the anti-H-2Kd monoclonal antibody SF1.1.1 [25].
• Immunoprecipitation experiments isolated a 48-kDa protein from a cell line transfected with a c-raf construct as well as from normal NIH 3T3 fibroblasts [14].

References