Rev (HIV)

Source: Wikipedia, the free encyclopedia.
REV protein bound to RRE mRNA
SCOP2
484d / SCOPe / SUPFAM
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary

Rev is a transactivating

mRNAs
. In the absence of Rev, mRNAs of the HIV-1 late (structural) genes are retained in the nucleus, preventing their translation.

History

A novel protein was found to be involved in the translation of gag and env mRNA. The unknown protein functioned by removing repression of regulatory sequences and was named Art (anti-repression transactivator).[1] Later studies suggested that the protein was involved in regulation of the RNA splicing mechanism. Therefore, the name of the protein was modified from Art to Trs (transregulator of splicing).[2] The most recent studies have shown that the protein has multiple functions in the regulation of HIV-1 proteins, and its name has been changed to Rev (regulator of expression of virion proteins), which more generally describes its function.[3]

Structure

Rev is a 13-kDa protein[4] that is composed of 116 amino acids.[5] Rev's sequence contains two specific domains which contribute to its nuclear import and export. The protein typically performs its function as a tetramer.[citation needed]

Arginine-rich motif

The

nuclear localization signal.[11]

Rev-activation domain (Nuclear export signal)

Rev's

C-terminal region[13] and is leucine-rich.[14] Binding of Rev to viral RNAs containing the RRE allows for mRNA export out of the nucleus and into the cytoplasm by a mechanism different than that of cellular mRNAs.[citation needed
]

Function

HIV-1 regulatory proteins (including Rev) are translated from completely processed mRNA transcripts, while structural proteins are translated from incompletely spliced transcripts. Completely spliced transcripts are exported from the nucleus to the cytoplasm by the same mechanism as cellular mRNA. However, Rev is needed to export incompletely spliced mRNAs in order to produce the viral structural proteins.[citation needed]

Rev localization to the nucleus

The arginine-rich domain of the Rev protein, containing a

Ran-GDP, and importin-β (a nuclear transport factor).[15] The Rev NLS is a highly similar sequence to that of the importin-β-binding site present within importin-α,[16] which allows for the interaction between Rev and importin-β. The NLS overlaps with the sequence required for RNA-binding.[17] This prevents the NLS from counteracting the export of RRE-containing mRNA transcripts.[citation needed]

The secondary structure of the IIB binding site shows non-canonical base pairs G47 (magenta)-A73 (orange) and G48-G71 (magenta). Bulging, non-paired uridine nucleotide points outward from the secondary helix (colored red). The mRNA forms a stem-loop like structure with intricate folding (PDB 4PMI).

Binding of Rev to the RRE

The rev response element (RRE) is a 240 base-pair sequence located in the second

HIV-1 genome, immediately downstream of the env gene.[18] The RRE remains functional if translocated, but needs to remain in the same orientation (cannot be inverted). The RRE is retained by incompletely processed mRNA transcripts. The secondary structure of the RRE creates eight stem-loops. Rev initially binds to the purine-rich stem-loop IIB,[19] then binds to a secondary site in stem-loop I.[20]

Within this purine-rich stem-loop, IIB, are non-canonical base pairs that form as a result of the mRNA stem loop-secondary structure. These base pairs include guanine-adenine (nucleotides 47 and 73, respectively) and guanine-guanine (nucleotides 48–71, respectively). The two base pairs are separated by a non-stacked and bulging uridine that points outwards, away from the ARM-RNA interactions.

Shown are residues Arg35 and Arg39 (colored by element with IUPAC standards) that make specific contacts with residues uracil 66 (red), guanine 67, and guanine 70 (magenta) during RNA binding (PDB 4PMI).

The ARM contains residues R35 and R39 that make base-specific contacts with residues on the RRE mRNA, specifically to bases uracil 66, guanine 67, and guanine 70, respectively. On the opposite side of these bases, residues N40 and R44 make base-specific contacts with nucleotides uracil 45, guanine 46, guanine 47, and adenine 73. In addition to these stabilizing contacts, additional Arg residues within the ARM, as well as T34, make nonspecific contacts with bases on the mRNA.[21]

Shown are residues N40 and R44 (colored by element with IUPAC standards) making specific contacts with residues uracil 45 (red), guanine 46 (magenta), guanine 47 (magenta), and adenine 73 (orange) (PDB 4PMI).

The RRE sequence is cis-acting, and is necessary to achieve high levels of env mRNA in the cytoplasm.[22] The RRE also facilitates multimerization of the Rev proteins, which is required for Rev binding and function.[23] The Rev protein binds unspliced gag and pol transcripts and incompletely spliced env, vif, vpr and vpu transcripts at the RRE, facilitating export to the cytoplasm.[24]

Genomic export from the nucleus

Rev is continuously shuttled between the

Ran-GTP at the RRE sequence within incompletely spliced transcripts. Following assembly of the complex, the intron-containing RNAs are exported from the nucleus into the cytoplasm.[25] Once the pre-mRNAs are in the cytoplasm, Rev dissociates, revealing the NLS.[26] Exposure of the NLS allows for Rev interaction with importin-β in order to shuttle Rev back to the nucleus.[citation needed
]

Rev-directed export of viral RNAs is similar to the mechanism by which

rRNAs are exported, as opposed to the mechanism for export of cellular mRNAs. Rev is able to facilitate export of pre-mRNA transcripts that would otherwise typically remain in the nucleus, suggesting that the Rev NES is dominant over nuclear retention.[27]

Regulation of HIV gene expression

Rev acts post-transcriptionally to positively regulate the expression of structural genes and to negatively regulate the expression of

virion production.[30]

Transition from early to late phase HIV-1 genes

regulatory genes) occurs in the same manner as the normal export of cellular mRNAs. On the other hand, unspliced and incompletely spliced mRNAs which code for the late, structural proteins are Rev-dependent. The Rev protein is expressed as an early gene from completely spliced transcripts, so the expression of late phase structural proteins cannot occur until an initial amount of Rev is produced.[31]

Rev as a target for antiviral therapies

Since Rev is absolutely necessary for HIV-1 replication and it is expressed early on in infection, it has been suggested that Rev is a good target for antiviral therapies.[citation needed]

virion
assembly, are not produced.

It has been shown that various organic compounds have the ability to target the Rev/RRE interaction.

cation, and proflavine are small molecules that can prevent Rev from binding to the RRE sequence.[34][35][36] If Rev is incapable of binding to the RRE on the pre-mRNA, the RNA will not be exported to the cytoplasm, also resulting in lack of necessary structural proteins.[citation needed
]

Other therapies target the Rev protein itself, since it is an essential component of

wild-type Rev protein for the RRE binding site, and therefore decrease Rev's normal cellular functions.[37]

Dihydrovaltrate was also identified as a Rev-export inhibitory congener.[citation needed]

References