The intronless human gene RSC1A1 on chromosome 1p36.1 encodes a 70 kDa protein called RS1 that is expressed in various tissues (1). RS1 contains several consensus sequences for protein kinase C, a consensus sequence for casein kinase II, and a C-terminal ubiquitin-associated domain. Upon confluence of renal epithelial LLC-PK1 cells, the expression of RS1 protein is down-regulated posttranscriptionally, and its intracellular distribution changes dramatically (2). In subconfluent LLC-PK1 cells, RS1 is located at the plasma membrane, at vesicles below the plasma membrane, at the trans-Golgi network, and within the nucleus. In confluent LLC-PK1 cells, RS1 was observed at the trans-Golgi but could not be detected at the plasma membrane or within the nucleus. Because the Na⁺-D-glucose cotransporter SGLT1 is – opposite to RS1 – up-regulated by confluence, and because up-regulation of SGLT1 was similarly observed after reducing the expression of RS1 by an antisense strategy, we investigated the role of RS1 in the transcriptional and posttranscriptional regulation of SGLT1. Run-off assays with nuclei from confluent LLC-PK1 cells showed that transcription of SGLT1 mRNA was increased when the expression of RS1 was reduced (2). This suggests that nuclear RS1 inhibits transcription of SGLT1. Posttranscriptional regulation of SGLT1 by RS1 was investigated in oocytes of Xenopus laevis (3). SGLT1 was expressed by cRNA injection and the transport activity of SGLT1 was determined as phlorizin-inhibitable uptake of radioactively labeled α-methyl-D-glucoside (AMG). Effects of RS1 on the expression of SGLT1 were tested in two ways: Either the cRNAs of RS1 and SGLT1 were injected together and oocytes were incubated 3 days for expression, or SGLT1 cRNA was injected, oocytes were incubated for 3 days, and purified RS1 protein was injected into the oocytes shortly before uptake of AMG was measured. Co-expression of cRNAs (human RS1 with human SGLT1) resulted in a 50-80% decrease of the expressed AMG uptake. This decrease was due to a decrease of SGLT1 expression because the V_max for AMG uptake was reduced whereas the functional characteristics of glucose transport were not changed (K_m value, sodium acitivation, and sugar specificity). Interestingly, injection of human RS1 protein produced a similar decrease of AMG uptake (50-80%). For the effect of RS1 protein a time period of 30 min was required. A functional characterization of the posttranscriptional inhibition of SGLT1 by RS1 revealed (i) that the effect of RS1 was dependent on the function of dynamin, (ii) that the effect of RS1 was stimulated by protein kinase C, (iii) that it persisted in the presence of inhibitors of endocytosis, and (iv) that the effect of RS1 was abolished after inhibition of exocytosis by botulinus toxin. The data strongly suggest that the posttranscriptional regulation of SGLT1 by RS1 involves modulation of fission of SGLT1 containing vesicles from the trans-Golgi network. RS1 is more broadly expressed compared to SGLT1 and inhibits a variety of other plasma membrane transporters in addition to SGLT1 (3). To elucidate the physiological importance of RS1, we removed the Rsc1A1 gene in mice and analyzed phenotype and expression of SGLT1 (4). In small intestine of Rsc1A1 knockout-mice a sevenfold posttranscriptional up-regulation of SGLT1 was observed that resulted in a twofold increase of small intestinal glucose reabsorption. Several months after birth, the mice developed obesity of the intestinal type. The data indicate that RS1 participates in the regulation of SGLT1 expression and plays a critical role for the regulation of glucose uptake in small intestine.