1exy
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SOLUTION STRUCTURE OF HTLV-1 PEPTIDE BOUND TO ITS RNA APTAMER TARGET
Structural highlights
FunctionREX_HTL1C Rex escorts unspliced gag-pro-pol and singly spliced env mRNAs out of the nucleus of infected cells. These mRNAs carry a recognition sequence called Rex responsive element (RxRE or XRE) located at the 3' region of the long terminal repeat (LTR). This function is essential since most HTLV proteins are translated from unspliced or partially spliced pre-mRNAs that cannot exit the nucleus by the pathway used by fully processed cellular mRNAs. Rex itself is translated from a fully spliced mRNA that probably readily exits the nucleus. Rex's nuclear localization signal (NLS) binds directly to KPNB1/importin beta-1 without previous binding to KPNA1/importin alpha-1. KPNB1 binds to the GDP bound form of RAN (Ran-GDP) and targets Rex to the nucleus. In the nucleus, the conversion from Ran-GDP to Ran-GTP dissociates Rex from KPNB1 and allows Rex's binding to the RRE in viral pre-mRNAs. Rex multimerizes on the RRE via cooperative assembly. This multimerization is critical for its full biological activity, since it may shield the viral RNA from being spliced or down-regulated, and probably exposes Rex's nuclear export signal (NES) to the surface. Rex can then form a complex with XPO1/CRM1, RANBP3 and Ran-GTP, leading to nuclear export of the complex. Conversion from Ran-GTP to Ran-GDP mediates dissociation of the Rex/RRE/XPO1/RANBP3/RAN complex, so that Rex can return to the nucleus for a subsequent round of export (By similarity). Publication Abstract from PubMedBACKGROUND: The Rex protein of the human T cell leukemia virus type 1 (HTLV-1) belongs to a family of proteins that use arginine-rich motifs (ARMs) to recognize their RNA targets. Previously, an in vitro selected RNA aptamer sequence was identified that mediates mRNA transport in vivo when placed in the primary binding site on stem-loop IID of the Rex response element. We present the solution structure of the HTLV-1 arginine-rich Rex peptide bound to its RNA aptamer target determined by multidimensional heteronuclear NMR spectroscopy. RESULTS: The Rex peptide in a predominantly extended conformation threads through a channel formed by the shallow and widened RNA major groove and a looped out guanine. The RNA aptamer contains three stems separated by a pair of two-base bulges, and adopts an unanticipated fold in which both junctional sites are anchored through base triple formation. Binding specificity is associated with intermolecular hydrogen bonding between guanidinium groups of three non-adjacent arginines and the guanine base edges of three adjacent G.C pairs. CONCLUSIONS: The extended S-shaped conformation of the Rex peptide, together with previous demonstrations of a beta-hairpin conformation for the bovine immunodeficiency virus (BIV) Tat peptide and an alpha-helical conformation for the human immunodeficiency virus (HIV) Rev peptide in complex with their respective RNA targets, expands our understanding of the strategies employed by ARMs for adaptive recognition and highlights the importance of RNA tertiary structure in accommodating minimalist elements of protein secondary structure. Anchoring an extended HTLV-1 Rex peptide within an RNA major groove containing junctional base triples.,Jiang F, Gorin A, Hu W, Majumdar A, Baskerville S, Xu W, Ellington A, Patel DJ Structure. 1999 Dec 15;7(12):1461-72. PMID:10647177[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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