Structure-function analyses of the HTLV-I Rex and HIV-1 Rev RNA response elements: insights into the mechanism of Rex and Rev action.

Abstract

The ability of the Rex protein of the type I human T-cell leukemia virus (HTLV-I) to regulate expression of the retroviral gag and env structural genes post-transcriptionally is critically dependent on the presence of a Rex response element (RexRE). This c/s-regulatory sequence is located within the retroviral 3' long terminal repeat and coincides with a predicted, highly stable RNA stem-loop structure. Rex action requires both the overall secondary structure intrinsic to the RexRE and specific sequences from one small subregion of this large structure. This small subregion likely forms a protein-binding site for Rex or a cellular RNA-binding factor. Whereas Rex can functionally replace the Rev protein of the type 1 human immunodeficiency virus (HIV-I} through its interaction with the analogous Rev response element (RevRE), distinct subregions of this HIV-1 RNA element mediate the responses to Rex and Rev. Strikingly, Rex acts as a dominant repressor of Rev action, following the deletion of the Rex responsive subregion of the RevRE. Similarly, Rev inhibits Rex function in a dominant manner when the Rev responsive subregion of the RevRE is deleted. Together, these findings suggest that Rex and Rev not only interact with their respective RNA response elements but also may either form mixed inactive multimers or interact with a common cellular factorIs). If binding of a common host protein is involved, this factor likely plays a central role either in spliceosome assembly or nuclear RNA transport.

The Human T-cell leukemia virus Rex protein

“…Both Rex-1 and Rex-2 have homologous nuclear localization signals (NLS), RNA binding domains (RBD), multimerization domains, and an activation domain which encompasses the nuclear export signal (NES) (48,(62)(63)(64)(65)(66). In addition, a unique C-terminal domain has been described for Rex-2 that is a target for serine phosphorylation and may also contribute to efficient nucleocytoplasmic shuttling (54).…”

Molecular and cellular aspects of HTLV-1 associated leukemogenesis in vivo

“…The rex responsive element (RXRE) is a cis-acting RNA element required for Rex function that maps to the U3R region of the LTR. [74][75][76][77][78] HTLV-1 Rex protein acts at the posttranscriptional level to induce the appearance of unspliced and singly spliced viral mRNA in the cytoplasm. 79 Rex action requires both the overall secondary structure intrinsic to the RXRE and specific sequences from one small region of this large structure 74,80,81 which forms a binding site for the protein.…”

“…Position 1 corresponds to position 8615 of the ATK sequence. 35 The three Rex binding motifs are boxed [74][75][76][77][78] …”

HIV-1 Nef-associated Factor 1 Enhances Viral Production by Interacting with CRM1 to Promote Nuclear Export of Unspliced HIV-1 gag mRNA

“…Rev contains a leucine-rich nuclear export signal (NES) domain located near the carboxyl terminus, through which Rev interacts with CRM1, and the Rev-RRE-CRM1/Ran-GTP complex is then transported to the cytoplasm (6, 11-16). Similarly, other retroviruses such as mouse mammary tumor virus and human T cell lymphotropic virus encode Rev-like regulatory proteins Rem or Rex, respectively, for hijacking the host CRM1 pathway to accomplish nuclear export of unspliced viral transcripts (17)(18)(19)(20)(21)(22).Several other cellular factors have also been identified as Rev co-factors that are required for efficient Rev function, such as the RNA binding motif protein 14, the single-stranded nucleic acid-binding protein Pur-alpha, eukaryotic initiation factor-5A, and the DEAD (Asp-Glu-Ala-Asp) box RNA helicase DDX1, DDX3, and DDX5 (23-31). However, some host factors recruited by Rev can interfere with its function.…”

“…Rev contains a leucine-rich nuclear export signal (NES) domain located near the carboxyl terminus, through which Rev interacts with CRM1, and the Rev-RRE-CRM1/Ran-GTP complex is then transported to the cytoplasm (6, 11-16). Similarly, other retroviruses such as mouse mammary tumor virus and human T cell lymphotropic virus encode Rev-like regulatory proteins Rem or Rex, respectively, for hijacking the host CRM1 pathway to accomplish nuclear export of unspliced viral transcripts (17)(18)(19)(20)(21)(22).…”