contributed equally to this work Mammalian TIF1α and TIF1β (KAP-1/KRIP-1) are related transcriptional intermediary factors that possess intrinsic silencing activity. TIF1α is believed to be a euchromatic target for liganded nuclear receptors, while TIF1β may serve as a co-repressor for the large family of KRAB domain-containing zinc finger proteins. Here, we report an association of TIF1β with both heterochromatin and euchromatin in interphase nuclei. Co-immunoprecipitation of nuclear extracts shows that endogenous TIF1β, but not TIF1α, is associated with members of the heterochromatin protein 1 (HP1) family. However, in vitro, both TIF1α and TIF1β interact with and phosphorylate the HP1 proteins. This interaction involves a conserved amino acid motif, which is critical for the silencing activity of TIF1β but not TIF1α. We further show that trichostatin A, an inhibitor of histone deacetylases, can interfere with both TIF1 and HP1 silencing. The silencing activity of TIF1α appears to result chiefly from histone deacetylation, whereas that of TIF1β may be mediated via both HP1 binding and histone deacetylation.

The ric-3 gene is required for maturation of nicotinic acetylcholine receptors in Caenorhabditis elegans. The human homolog of RIC-3, hRIC-3, enhances expression of ␣7 nicotinic receptors in Xenopus laevis oocytes, whereas it totally abolishes expression of ␣4␤2 nicotinic and 5-HT 3 serotonergic receptors. Both the N-terminal region of hRIC-3, which contains two transmembrane segments, and the C-terminal region are needed for these differential effects. hRIC-3 inhibits receptor expression by hindering export of mature receptors to the cell membrane. By using chimeric proteins made of ␣7 and 5-HT 3 receptors, we have shown that the presence of an extracellular isoleucine close to the first transmembrane receptor fragment is responsible for the transport arrest induced by hRIC-3. Enhancement of ␣7 receptor expression occurs, at least, at two levels: by increasing the number of mature receptors and facilitating its transport to the membrane. Certain amino acids of a putative amphipathic helix present at the large cytoplasmic region of the ␣7 subunit are required for these actions. Therefore, hRIC-3 can act as a specific regulator of receptor expression at different levels.

The expression of several genes involved in intra-and extracellular lipid metabolism, notably those involved in peroxisomal and mitochondrial ␤-oxidation, is mediated by ligand-activated receptors, collectively referred to as peroxisome proliferator-activated receptors (PPARs). To gain more insight into the control of expression of carnitine palmitoyltransferase (CPT) genes, which are regulated by fatty acids, we have examined the transcriptional regulation of the human MCPT I gene. We have cloned by polymerase chain reaction the 5-flanking region of this gene and demonstrated its transcriptional activity by transfection experiments with the CAT gene as a reporter. We have also shown that this is a target gene for the action of PPARs, and we have localized a PPAR responsive element upstream of the first exon. These results show that PPAR regulates the entry of fatty acids into the mitochondria, which is a crucial step in their metabolism, especially in tissues like heart, skeletal muscle and brown adipose tissue in which fatty acids are a major source of energy.The incorporation of activated long-chain fatty acids into the mitochondria to be catabolized through ␤-oxidation is produced by the mitochondrial carnitine palmitoyltransferase (CPT) 1 enzyme system. CPT I, the outer membrane component of this system, is the main control point in the ␤-oxidation pathway. CPT I is thus a suitable site for pharmacological control of fatty acid oxidation in conditions such as diabetes or heart diseases.Two isoforms of CPT I have been described, which have been designated LCPT I and MCPT I since these isoforms are mainly expressed in liver and muscle respectively. The MCPT I gene is expressed not only in skeletal muscle but also in heart and brown and white adipose tissue (1-4). This expression pattern may be of great significance since fatty acids are a major source of energy for heart, skeletal muscle, and brown adipose tissue.The CPT I gene expression is regulated by fatty acids and peroxisome proliferators (5, 6). To gain more insight into the control of CPT I gene expression by fatty acids, we have examined the transcriptional regulation of CPT I genes. The expression of several genes involved in intra-and extracellular lipid metabolism, notably those involved in peroxisomal and mitochondrial ␤-oxidation, is mediated by ligand-activated receptors collectively referred to as peroxisome proliferator-activated receptors (PPARs); these receptors are members of the nuclear receptor superfamily. PPARs are activated by a wide array of peroxisome proliferators and also by natural and synthetic fatty acids (7,8), antidiabetic drugs (9, 10), prostaglandin J 2 (10), and leukotriene B 4 (11).We have amplified by polymerase chain reaction (PCR) the 5Ј region of the human heart and brown adipose tissue CPT I gene and demonstrate, first, the transcriptional activity of this fragment and, second, the presence of a PPRE in the 5Ј-flanking region of this gene. In CV1 cells, the activation of the CPT I gene by PPAR was dependent on the ad...