We have identified between Mex67p and Mtr2p a complex which is essential for mRNA export. This complex, either isolated from yeast or assembled in Escherichia coli, can bind in vitro to RNA through Mex67p. In vivo, Mex67p requires Mtr2p for association with the nuclear pores, which can be abolished by mutating either MEX67 or MTR2. In all cases, detachment of Mex67p from the pores into the cytoplasm correlates with a strong inhibition of mRNA export. At the nuclear pores, Nup85p represents one of the targets with which the Mex67p-Mtr2p complex interacts. Thus, Mex67p and Mtr2p constitute a novel mRNA export complex which can bind to RNA via Mex67p and which interacts with nuclear pores via Mtr2p.Transport through nuclear pores requires concerted action between the structural components of the nuclear pore complex (NPC) and the soluble transport factors that bind to the transport substrates and shuttle between the nuclear and cytoplasmic compartments (for reviews, see references 2 and 31). Substantial progress toward an understanding of nuclear protein import has been achieved in the past few years, but very little is known about how RNA is exported from the nucleus into the cytoplasm. Among the factors required for nuclear protein import are the classical nuclear localization signalreceptor complex, consisting of importin/karyopherin ␣ and ␤, the small GTPase Ran, and several Ran-binding proteins, as well as repeat sequences containing nucleoporins (for reviews, see references 5 and 11). Recently, additional routes of import into the nucleus were discovered, suggesting that major classes of transport substrates use different import pathways. Transportin and Kap123p were identified as novel transport factors that bind directly to their transport substrates, hnRNP protein A1 and ribosomal protein L25, respectively (30, 38). Transportin and Kap123p belong to a growing family of importin ␤-like proteins which have a Ran GTP-binding domain in their amino-terminal portions (5, 10). Recently, Mtr10p, which is also a member of this protein family, was shown to be the importin for yeast Np13p (34,41). An essential role for Ran in energydependent nuclear protein import has been firmly established, but how Ran and the many Ran activity-modulating proteins participate in the actual translocation process is still controversial.The Ran system is also involved in transport from the nucleus (9,14,18,36). It has been firmly established that nuclear export sequences (NES), first identified in viral proteins such as human immunodeficiency virus Rev and protein kinase inhibitor, mediate the exit of proteins from the nucleus (for a review, see reference 8). For the Rev protein, which is an RNA-binding protein with a specificity for unspliced or partially spliced viral transcripts, viral mRNA is coexported through association with Rev (3). Initially, it was found that Rev NES interact with Rip (6,47), which resembles repeat sequence-containing nucleoporins and accordingly was suggested to be a NES receptor at the nuclear pores. Recen...

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We have reported a novel functional co-operation among MyoD, myocyte enhancer factor-2 (MEF2), and the thyroid hormone receptor in a muscle-specific enhancer of the rat GLUT4 gene in muscle cells. Here, we demonstrate that the muscle-specific enhancer of the GLUT4 gene operates in skeletal muscle and is muscle fiber-dependent and innervation-independent. Under normal conditions, both in soleus and in extensor digitorum longus muscles, the activity of the enhancer required the integrity of the MEF2-binding site. Cancellation of the binding site of thyroid hormone receptor enhanced its activity, suggesting an inhibitory role. Muscle regeneration of the soleus and extensor digitorum longus muscles caused a marked induction of GLUT4 and stimulation of the enhancer activity, which was independent of innervation. During muscle regeneration, the enhancer activity was markedly inhibited by cancellation of the binding sites of MEF2, MyoD, or thyroid hormone receptors. Different MEF2 isoforms expressed in skeletal muscle (MEF2A, MEF2C, and MEF2D) and all members of the MyoD family had the capacity to participate in the activity of the GLUT4 enhancer as assessed by transient transfection in cultured cells. Our data indicate that the GLUT4 enhancer operates in muscle fibers and its activity contributes to the differences in GLUT4 gene expression between oxidative and glycolytic muscle fibers and to the GLUT4 up-regulation that occurs during muscle regeneration. The activity of the enhancer is maintained in adult muscle by MEF2, whereas during regeneration the operation of the enhancer depends on MEF2, myogenic transcription factors of the MyoD family, and thyroid hormone receptors.

Cell identity is maintained by activation of cell-specific gene programs, regulated by epigenetic marks, transcription factors and chromatin organization1-3. DNA G-quadruplex (G4)-folded regions in cells were reported to be associated with either increased or decreased transcriptional activity4,5. By G4 ChIP-seq/RNA-seq analysis on liposarcoma cells we confirmed that G4s in promoters are invariably associated with high transcription levels in open chromatin. Comparing G4 presence, location and transcript levels in liposarcoma cells to available data on keratinocytes, we showed that the same promoter sequences of the same genes in the two cell lines had different G4-folding state: high transcript levels consistently associated with high G4-folding. Transcription factors AP-1 and SP1, whose binding sites were the most significantly represented in G4-folded sequences, coimmunoprecipitated with their G4-folded promoters. Thus G4s and their associated transcription factors cooperate to determine cell-specific transcriptional programs, making G4s strongly emerge as new epigenetic regulators of the transcription machinery.