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RS1 is the intron less singel copy gene involved in regulation of plasme membrane transporters. Ornithine decarboxylase is identified as the receptor of RS1 specific for the release of vesicles containing SGLT1 specifically at the trans-golgi network. RS1 decreases the activity of ODC there by inhibiting the release of vesicles containing specifically SGLT1.
The RS1 protein (gene RSC1A1) participates in regulation of Na+-D-glucose cotransporter SGLT1 and some other solute carriers. In subconfluent LLC-PK1 cells, RS1 inhibits release of SGLT1 from the trans-Golgi network and transcription of SGLT1. In subconfluent cells, RS1 is localized in the nucleus and the cytoplasm whereas confluent cells contain predominantly cytoplasmic RS1. In the present study, the mechanism and regulation of confluence-dependent nuclear location of RS1 was investigated. Confluence dependent nuclear location of RS1 was shown to be regulated by the cell cycle. A nuclear shuttling signal (NS) in pRS1 was identified that ensures confluence-dependent distribution of pRS1 and comprises nuclear localization signal (NLS) and nuclear export signal (NES). The NLS and NES of RS1 mediate translocation into and out of the nucleus via importin ß1 and CRM1, respectively, and the nuclear/cytoplasmic distribution of the RS1 protein is determined by the nuclear export activity. The adjacent protein kinase C (PKC) phosphorylation site at serine 370 of pRS1 was shown to control nuclear localization driven by NS and is necessary for the differential localization of RS1 in quiescent versus proliferating cells. Basing on the data of site-directed mutagenesis, PKC activation experiments and mass spectrometry analysis of RS1 phosphorylation, the following model of the regulation of RS1 nuclear location in LLC-PK1 cells was proposed. In subconfluent cells, RS1 is actively imported into the nucleus whereas nuclear export of RS1 is not active leading to accumulation of RS1 in the nucleus. After confluence, phosphorylation of serine 370 of pRS1 by PKC takes place leading to enhancement of RS1 nuclear export and predominantly cytoplasmic distribution of the protein in the confluent cells. The confluence-dependent regulation of RS1 localization may control SGLT1 expression during regeneration of enterocytes in small intestine and during regeneration of renal tubular cells after hypoxemic stress. Moreover, the gene expression profiling of mouse embryonic fibroblasts with RS1-/- genotype suggests that transcriptional regulation by RS1 might be important for the cell cycle and cell division. Since RS1 localization depends on the cell cycle, RS1 might play a role in the regulation of the solute carriers during specific phases of the cell cycle.
The RS1 protein, a 67 kDa protein, encoded by an intronless single copy gene that was only detected in mammals, mediates transcriptional and post-transcriptional down-regulation of the sodium-D-glucose co-transporter SGLT1. The short-term post-transcriptional down-regulation of SGTL1 by RS1 has been shown to occur at the trans-Golgi network (TGN). In the present study, two tripeptides from the human RS1 protein (hRS1), GlnCysPro and GlnSerPro, that induce the post-transcriptional down-regulation of SGLT1 at the TGN, were identified. The application of the tripeptides led to 40-50% reduction of the amount of the SGLT1 protein in the plasma membrane, which correlated to the degree of decrease in SGLT1-mediated glucose transport. For the short-term down-regulation of SGLT1 by the tripeptides, the effective intracellular concentrations IC50 values of 2.0 nM (GlnCysPro, QCP) and 0.16 nM (GlnSerPro, QSP) were estimated. The observed down-regulation of SGLT1 by the tripeptides QCP and QSP, similar to hRS1 protein, was attenuated by different intracellular monosaccharides including nonmetabolized methyl-α-D-glucopyranoside and 2-deoxyglucose. On the contrary, the short-term inhibition of the hOCT2 by QCP could only be observed after rising of intracellular concentration of AMG. QCP and QSP are transported by H+-peptide cotransporter PEPT1 that is co-located with SGLT1 in the small intestinal enterocytes and thereafter effectively down-regulate hSGLT1-mediated transport of AMG. The data indicates that orally applied tripeptides QCP or QSP can be used to down-regulate D-glucose absorption in small intestine and used for treatment of obesity and diabetes mellitus.