The ob gene product, leptin, is an important circulating signal for the regulation of body weight. To identify high affinity leptin-binding sites, we generated a series of leptin-alkaline phosphatase (AP) fusion proteins as well as [125I]leptin. After a binding survey of cell lines and tissues, we identified leptin-binding sites in the mouse choroid plexus. A cDNA expression library was prepared from mouse choroid plexus and screened with a leptin-AP fusion protein to identify a leptin receptor (OB-R). OB-R is a single membrane-spanning receptor most related to the gp130 signal-transducing component of the IL-6 receptor, the G-CSF receptor, and the LIF receptor. OB-R mRNA is expressed not only in choroid plexus, but also in several other tissues, including hypothalamus. Genetic mapping of the gene encoding OB-R shows that it is within the 5.1 cM interval of mouse chromosome 4 that contains the db locus.

The leptin receptor (OB-R) is a single membrane-spanning protein that mediates the weight regulatory effects of leptin (OB protein). The mutant allele (db) of the OB-R gene encodes a protein with a truncated cytoplasmic domain that is predicted to be functionally inactive. Several mRNA splice variants encoding OB-Rs with different length cytoplasmic domains have been detected in various tissues. Here we demonstrate that the full-length OB-R (predominantly expressed in the hypothalamus), but not a major naturally occurring truncated form or a mutant form found in db/db mice, can mediate activation of signal transducer and activator of transcription (STAT) proteins and stimulate transcription through interleukin 6 responsive gene elements. Reconstitution experiments suggest that, although OB-R mediates intracellular signals with a specificity similar to interleukin 6-type cytokine receptors, signaling appears to be independent of the gpl3O signal transducing component of the interleukin 6-type cytokine receptors.

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Uncoupling protein 1 (UCP1) dissipates energy and generates heat by catalyzing back-flux of protons into the mitochondrial matrix, probably by a fatty acid cycling mechanism. If the newly discovered UCP2 and UCP3 function similarly, they will enhance peripheral energy expenditure and are potential molecular targets for the treatment of obesity. We expressed UCP2 and UCP3 in Escherichia coli and reconstituted the detergent-extracted proteins into liposomes. Ion flux studies show that purified UCP2 and UCP3 behave identically to UCP1. They catalyze electrophoretic flux of protons and alkylsulfonates, and proton flux exhibits an obligatory requirement for fatty acids. Proton flux is inhibited by purine nucleotides but with much lower affinity than observed with UCP1. These findings are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell.Uncoupling protein 1 (UCP1) 1 of brown adipose tissue mitochondria occupies a special place in bioenergetics, because it is the exception that proves the rule of Mitchell's elegant chemiosmotic theory (1), a protein designed to short circuit the redox proton pumps in order to generate heat and dissipate energy. UCP1 was identified from functional studies on brown adipose tissue mitochondria (2) and was one of the first membrane proteins to be sequenced (3).For many years it was thought that UCP was expressed solely in mammalian brown adipose tissue; however, it now turns out that Nature has engineered at least five uncoupling proteins. In 1995, a plant uncoupling protein was discovered and later sequenced (4, 5), and 2 years later, UCP2 and UCP3 were identified (6 -9). UCP4 was recently described as a brainspecific UCP (10). UCP2 maps to regions of human chromosome 11 and mouse chromosome 7 that have been linked to hyperinsulinemia and obesity, and it is hypothesized that UCP2 is the peripheral target for energy dissipation in the regulation of body weight. UCP2 is ubiquitously expressed in mammalian tissues, whereas UCP3 is expressed primarily in glycolytic skeletal muscle in humans and may account for the thermogenic effect of thyroid hormone (11). These aspects of this rapidly emerging area of research have been nicely reviewed by Boss et al. (12).Virtually nothing is known about the transport functions of UCP2 and UCP3, and their putative physiological functions have been deduced primarily from their striking sequence identities with UCP1 (12). To address this problem, we expressed human UCP2 and UCP3 in Escherichia coli, where they accumulated in inclusion bodies. Following detergent extraction, we reconstituted the proteins into liposomes and measured H ϩ and K ϩ fluxes. Purified UCP2 and UCP3 both catalyzed electrophoretic flux of protons and alkylsulfonates, and proton flux exhibited an obligatory requirement for fatty acids. We also found that FA-dependent proton transport by UCP2 and UCP3 was inhibited by purine nucleotides, albeit with lower apparent affinities for nucleotides than those observed with UCP1. From these results, we co...