Biomedical subjects

B Ossareh-Nazari

Publications and source records attributed to B Ossareh-Nazari.

6 recordsLinked to original sources

PubMed2001

NXT1 is necessary for the terminal step of Crm1-mediated nuclear export.

Soluble factors are required to mediate nuclear export of protein and RNA through the nuclear pore complex (NPC). These soluble factors include receptors that bind directly to the transport substrate and regulators that determine the assembly state of receptor-substrate complexes. We recently reported the identification of NXT1, an NTF2-related export factor that stimulates nuclear protein export in permeabilized cells and undergoes nucleocytoplasmic shuttling in vivo (Black, B.E., L. Lévesque, J.M. Holaska, T.C. Wood, and B.M. Paschal. 1999. Mol. Cell. Biol. 19:8616-8624). Here, we describe the molecular characterization of NXT1 in the context of the Crm1-dependent export pathway. We find that NXT1 binds directly to Crm1, and that the interaction is sensitive to the presence of Ran-GTP. Moreover, mutations in NXT1 that reduce binding to Crm1 inhibit the activity of NXT1 in nuclear export assays. We show that recombinant Crm1 and Ran are sufficient to reconstitute nuclear translocation of a Rev reporter protein from the nucleolus to an antibody accessible site on the cytoplasmic side of the NPC. Further progress on the export pathway, including the terminal step of Crm1 and Rev reporter protein release, requires NXT1. We propose that NXT1 engages with the export complex in the nucleoplasm, and that it facilitates delivery of the export complex to a site on the cytoplasmic side of NPC where the receptor and substrate are released into the cytoplasm.

Active Transport, Cell Nucleus↗

PubMed2001

Protein export from the nucleus.

The evolution of the nucleus imposed on eukaryotic cells the necessity to strictly control exchange of molecules between the nucleus and the remainder of the cell, not only to protect and correctly transmit genetic information, but also to coordinate nuclear and cytoplasmic functions. Studies over the past 10 years have provided major insights into the molecular mechanisms responsible for transport of molecules between the nucleus and the cytoplasm. In addition, regulation of the nucleocytoplasmic distribution of diverse cellular factors has emerged as one of the most efficient mechanism to adapt gene expression to the cell environment, for example by controlling the access of transcriptional regulators to their target genes. In this review, we focus on the molecular basis of protein nuclear export that relies on interactions between targeting sequences present on the cargoes, specific export receptors or exportins and nuclear pore proteins, with special emphasis on the role of the Ran GTPase and associated proteins in this process.

Active Transport, Cell Nucleus↗

PubMed2000

RanGTP-binding protein NXT1 facilitates nuclear export of different classes of RNA in vitro.

To better characterize the mechanisms responsible for RNA export from the nucleus, we developed an in vitro assay based on the use of permeabilized HeLa cells. This new assay supports nuclear export of U1 snRNA, tRNA, and mRNA in an energy- and Xenopus extract-dependent manner. U1 snRNA export requires a 5' monomethylated cap structure, the nuclear export signal receptor CRM1, and the small GTPase Ran. In contrast, mRNA export does not require the participation of CRM1. We show here that NXT1, an NTF2-related protein that binds directly to RanGTP, strongly stimulates export of U1 snRNA, tRNA, and mRNA. The ability of NXT1 to promote export is dependent on its capacity to bind RanGTP. These results support the emerging view that NXT1 is a general export factor, functioning on both CRM1-dependent and CRM1-independent pathways of RNA export.

Animals↗

PubMed1999

Domains of Crm1 involved in the formation of the Crm1, RanGTP, and leucine-rich nuclear export sequences trimeric complex.

Nuclear export of proteins containing a leucine-rich nuclear export sequence (NES) is mediated by a specific NES receptor known as Crm1. This protein, which is related to the karyopherin beta family, interacts directly with NES in a RanGTP-dependent manner. To characterize the domains of Crm1 involved in formation of the trimeric Crm1-NES-RanGTP complex, N- and C-terminal deletion mutants of Crm1 were generated and their ability to bind NES and RanGTP in vitro was analyzed. Our results indicate that two regions of Crm1 are required for the formation of the trimeric Crm1-NES-RanGTP complex, the N-terminal domain of Crm1 and the central domain of the receptor, starting after residue 160 with an essential region between 566 and 720. The N-terminal domain is homologous to the RanGTP-binding domain of karyopherin beta and therefore is likely involved in the interaction with RanGTP. Consequently, the central domain likely corresponds to the NES-binding site of Crm1.

Amino Acid Sequence↗

PubMed1999

Nuclear transport and transcriptional regulation.

Studies over the past 10 years have provided major insights into the molecular mechanisms responsible for active transport of macromolecules in and out of the nucleus. Nucleocytoplasmic transport pathways correspond to active and signal-mediated processes that involve substrates, adaptors and receptors. Regulation of both nuclear import and nuclear export is mainly exerted at the level of transport complex formation and has emerged as one of the most efficient mechanisms to adapt gene expression to the cell environment by restricting the access of transcriptional regulators to their target genes.

Animals↗

PubMed1997

Evidence for a role of CRM1 in signal-mediated nuclear protein export.

Chromosome maintenance region 1 (CRM1), a protein that shares sequence similarities with the karyopherin beta family of proteins involved in nuclear import pathway, was shown to form a complex with the leucine-rich nuclear export signal (NES). This interaction was inhibited by leptomycin B, a drug that prevents the function of the CRM1 protein in yeast. To analyze the role of the CRM1-NES interaction in nuclear export, a transport assay based on semipermeabilized cells was developed. In this system, which reconstituted NES-, cytosol-, and energy-dependent nuclear export, leptomycin B specifically blocked export of NES-containing proteins. Thus, the CRM1 protein could act as a NES receptor involved in nuclear protein export.

Adenosine Triphosphate↗