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  • Unlike CRM exportins Cse and Xpot

    2019-09-11

    Unlike CRM1, exportins Cse1 and Xpot lack a C-extension and instead rely primarily on changes in HEAT-repeat conformation and direct Ran-cargo contacts to ensure low cargo-binding affinity in the cytosol and cooperative assembly of the exportin/Ran/cargo complex in the nucleus (Cook et al., 2005, Cook et al., 2009, Matsuura and Stewart, 2004). Why does CRM1 operate differently? CRM1 is unique among structurally characterized β-karyopherins in recognizing cargos via its outer rather than inner surface. In addition, unlike the extensive interfaces seen in other exportin-cargo complexes, NES recognition by CRM1 more closely resembles an enzyme-substrate interaction in which a narrow surface cleft binds a peptide ligand. These differences contribute to CRM1’s versatility as an exportin, because the modular NES peptide is sufficient to confer cargo status to any protein and because an external binding site imposes fewer steric constraints on cargo binding. However, the trade-off is a loss of mechanical robustness: only a modest (2 Å) shift in helix position differentiates high-affinity from low-affinity binding, and a cleft-like binding site on the convex solenoid surface would be susceptible to fluctuations in HEAT-repeat conformation facilitating such a shift. Our data suggest that, by bridging across the solenoid and rigidifying the HEAT repeats, the C-extension reduces the amplitude of such fluctuations and helps prevent spontaneous opening of the NES-binding groove. Notably absent from our understanding of how CRM1 operates are the changes in conformation induced by NPC components and CRM1 cofactors such as RanBP3 (Englmeier et al., 2001, Lindsay et al., 2001), NLP1 (Waldmann et al., 2012), and Nup98 (Oka et al., 2010). An attractive farnesyl diphosphate synthase is that the C-extension and HEAT-9 loop are the molecular “handles” by which these factors manipulate CRM1 to regulate export activity. Indeed, RanBP3 may at least partly enhance CRM1’s Ran-binding activity by stabilizing the Ran-binding conformation of the HEAT-9 loop (Langer et al., 2011). Whether additional components of the nuclear transport machinery exert higher levels of regulation by modulating dynamic or conformational aspects of CRM1 export complex assembly is a question for future study.
    Experimental Procedures
    Acknowledgments
    Introduction DEAD (Asp-Glu-Ala-Asp)-box polypeptide 3, DDX3, is an ATP-dependent RNA helicase essential for Human Immunodeficiency virus type-1 (HIV-1) gene expression [1], [2]. DDX3 was first reported as a host co-factor involved in the nuclear export of Rev-dependent transcripts [3]. As such, DDX3 was shown to directly interact with CRM1 in a NES- and RanGTP-independent manner [3]. The interaction between DDX3 and CRM1 was shown to occur at the cytoplasmic face of the nuclear pore complex suggesting that the RNA helicase was probably assisting the late cytoplasmic steps of nuclear export [3]. Several reports have also shown the involvement of DDX3 in translation of the HIV-1 unspliced mRNA [4], [5], [6], [7]. During this process, the RNA helicase prepares the unspliced mRNA for translation initiation by acting on the TAR RNA motif [6], [7]. As such, destabilization of TAR is essential for cap structure recognition by the eIF4F complex and the subsequent recruitment of the 40S ribosomal subunit [6]. Interestingly, this process seems to occur in cytoplasmic granules defined as a pre-translation initiation intermediate in which the unspliced mRNA accumulates together with DDX3 and a subset of translation initiation factors including eIF4GI and PABPC1 [5]. Since DDX3 is an essential host factor required to promote nuclear export and translation of HIV-1 mRNAs [3], [5], [6], [7], it represents an interesting potential therapeutic target aimed to avoid viral resistance [8], [9], [10]. However, DDX3 has been involved in several steps of RNA metabolism and therefore, associated with different physiological processes including cell cycle progression, innate immune response and cancer [1], [2], [11], [12]. Therefore, a thorough understanding on the functions and mechanisms of action of DDX3 is critical for the development of novel and safer antiviral drugs targeting this enzyme.