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Stem cells from bone marrow, skeletal muscle and possibly other tissues can be identified by the 'side-population' (SP) phenotype. Although it has been assumed that expression of ABC transporters is responsible for this phenotype, the specific molecules involved have not been defined. Here we show that expression of the Bcrp1 (also known as Abcg2 murine/ ABCG2 human) gene is a conserved feature of stem cells from a wide variety of sources. Bcrp1 mRNA was expressed at high levels in primitive murine hematopoietic stem cells, and was sharply downregulated with differentiation. Enforced expression of the ABCG2 cDNA directly conferred the SP phenotype to bone-marrow cells and caused a reduction in maturing progeny both in vitro and in transplantation-based assays. These results show that expression of the Bcrp1 / ABCG2 gene is an important determinant of the SP phenotype, and that it might serve as a marker for stem cells from various sources.

β-Arrestins are proteins that bind phosphorylated heterotrimeric GTP-binding protein (G protein)-coupled receptors (GPCRs) and contribute to the desensitization of GPCRs by uncoupling the signal transduction process. Resensitization of GPCR responsiveness involves agonist-mediated receptor sequestration. Overexpression of β-arrestins in human embryonic kidney cells rescued the sequestration of $\\beta_2$-adrenergic receptor ($\\beta_2$AR) mutants defective in their ability to sequester, an effect enhanced by simultaneous overexpression of β-adrenergic receptor kinase 1. Wild-type $\\beta_2$AR sequestration was inhibited by the overexpression of two β-arrestin mutants. These findings suggest that β-arrestins play an integral role in GPCR internalization and thus serve a dual role in the regulation of GPCR function.