Identification of a domain in human immunodeficiency virus type 1 rev that is required for functional activity and modulates association with subnuclear compartments containing splicing factor SC35

Nuclear Import of the HIV-1 Core Precedes Reverse Transcription and Uncoating

HIV-1 reverse transcription (RT) occurs before or during uncoating, but the cellular compartment where RT and uncoating occurs is unknown. Using imaging and biochemical assays to track HIV-1 capsids in the nucleus during infection, we demonstrated that higher-order capsid complexes and/or complete cores containing the viral genome are imported into the nucleus. Inhibition of RT does not prevent capsid nuclear import; thus, RT may occur in nuclear compartments. Cytosolic and nuclear fractions of infected cells reveal that most RT intermediates are enriched in nuclear fractions, suggesting that HIV-1 RT occurs in the nucleus alongside uncoating. In agreement, we find that capsid in the nucleus induces recruitment of cleavage and polyadenylation specific factor 6 (CPSF6) to SC35 nuclear speckles, which are highly active transcription sites, suggesting that CPSF6 through capsid is recruiting viral complexes to SC35 speckles for the occurrence of RT. Thus, nuclear import precedes RT and uncoating, which fundamentally changes our understanding of HIV-1 infection.

Selyutina et al. show that HIV-1 cores containing the viral genome are imported into the nucleus for reverse transcription and uncoating. HIV-1 cores in the nucleus are recruited by CPSF6 to SC35 highly active transcription domains for viral reverse transcription, integration, and/or expression.

Intermolecular masking of the HIV-1 Rev NLS by the cellular protein HIC: novel insights into the regulation of Rev nuclear import

BACKGROUND

The HIV-1 regulatory protein Rev, which is essential for viral replication, mediates the nuclear export of unspliced viral transcripts. Rev nuclear function requires active nucleocytoplasmic shuttling, and Rev nuclear import is mediated by the recognition of its Nuclear Localisation Signal (NLS) by multiple import factors, which include transportin and importin β. However, it remains unclear which nuclear import pathway(s) predominate in vivo, and the cellular environment that modulates Rev nucleocytoplasmic shuttling remains to be characterised.

RESULTS

In our study, we have identified the cellular protein HIC (Human I-mfa domain-Containing protein) as a novel interactor of HIV-1 Rev. We demonstrate that HIC selectively interferes with Rev NLS interaction with importin β and impedes its nuclear import and function, but does not affect Rev nuclear import mediated by transportin. Hence, the molecular determinants mediating Rev-NLS recognition by importin β and transportin appear to be distinct. Furthermore, we have employed HIC and M9 M, a peptide specifically designed to inhibit the transportin-mediated nuclear import pathway, to characterise Rev nuclear import pathways within different cellular environments. Remarkably, we could show that in 293T, HeLa, COS7, Jurkat, U937, THP-1 and CEM cells, Rev nuclear import is cell type specific and alternatively mediated by transportin or importin β, in a mutually exclusive fashion.

CONCLUSIONS

Rev cytoplasmic sequestration by HIC may represent a novel mechanism for the control of Rev function. These studies highlight that the multivalent nature of the Rev NLS for different import receptors enables Rev to adapt its nuclear trafficking strategy.

Phosphorylation of RIG-I by casein kinase II inhibits its antiviral response

RIG-I is an intracellular RNA virus sensor that mediates a signaling pathway that triggers the alpha/beta interferon (IFN-α/β) immune defenses. However, the mechanism for regulation of RIG-I activity remains largely unknown. Here we show that RIG-I activity is regulated by phosphorylation and dephosphorylation in its repressor domain (RD). Threonine at amino acid (aa) 770 and serine at aa 854 to 855 of RIG-I are phosphorylated by casein kinase II (CK2) in the resting state of the cell and dephosphorylated when cells are infected by RNA virus. Mutation at aa position 770 or 854 to 855 of RIG-I renders it constitutively active. Pharmacological inhibition of CK2 enhances virus-induced expression of IFN-β and suppresses virus proliferation, while inhibition of phosphatase reduces virus-induced expression of IFN-β. Overexpression of CK2 suppresses RIG-I-mediated signaling, while silencing of CK2 results in the increased suppression of virus proliferation. Our results reveal a novel mechanism of the regulation of RIG-I activity during RNA virus infection.

HIV-1 Rev transactivator: a beta-subunit directed substrate and effector of protein kinase CK2

The phosphorylation of HIV-1 Rev by protein kinase CK2 is strictly dependent on the regulatory beta subunit of the kinase and is deeply affected by conformational changes of the substrate outside the phosphorylation site. Here we show that Rev modulates a variety of CK2 properties, including autophosphorylation, catalytic activity toward calmodulin, and susceptibility to polycationic effectors, whose common denominator is the involvement of the beta subunit. Rev's two major CK2 sites are located at its N-terminus, immediately adjacent to a helix-loop-helix motif. By comparing the behaviour of full-size Rev with that of synthetic peptides reproducing, with suitable modifications, its N-terminal 26 amino acids including the phosphoacceptor site (Ser 5, Ser 8) and amphipathic helix-1, it appears that the functional interaction of the N-terminal portion of Rev with the N-terminal domain of the beta subunit must rely on both electrostatic and hydrophobic interactions. The former mainly involve Rev's arginine-rich domain (residues 35-50) in helix-2, while the latter are mostly mediated by residues 12-24 of helix-1. These data disclose the possibility that, besides displaying protective, regulatory and targeting properties with respect to the catalytic subunit, the CK2 beta subunit also plays a role as a docking site for a subset of CK2 substrates.

An N-terminal arginine-rich cluster and a proline-alanine-threonine repeat region determine the cellular localization of the herpes simplex virus type 1 ICP34.5 protein and its ligand, protein phosphatase 1

The ICP34.5 protein facilitates herpes simplex virus replication by binding and activating protein phosphatase 1 (PP1) by means of a very conserved C-terminal GADD34-like region. Natural variants of the ICP34.5 differing in the number of arginines in an Arg-rich cluster at the N terminus and the number of Pro-Ala-Thr repeats in the central bridge region of the protein were cloned as fusion proteins with a reporter peptide (c-Myc or hrGFP) at the C terminus. The natural variants were obtained from strains differing in passage history, tissue culture behavior, and neuroinvasive disease potential. In transfected cells, these variants localized to different subcellular compartments. The N-terminal Arg-rich cluster acted as a cellular localization signal for discrete regions of the nucleus and cytoplasm, but the ultimate location of ICP34.5 was determined by the number of Pro-Ala-Thr repeats in the central bridge region. PP1 colocalized with the ICP34.5 variant in cells expressing the ICP34.5. The ICP34.5-mediated, herpes simplex virus strain-dependent differences in the modulation of PP1 location and function may be responsible for the strain-associated differences in tissue culture behavior and virulence of the virus.

Recruitment of the Crm1 nuclear export factor is sufficient to induce cytoplasmic expression of incompletely spliced human immunodeficiency virus mRNAs

Cytoplasmic expression of the incompletely spliced RNA transcripts that encode the late, structural proteins of human immunodeficiency virus type 1 (HIV-1) is dependent on the viral Rev regulatory protein. General agreement exists that Rev acts, at least in part, by recruiting the cellular Crm1 nuclear export factor to HIV-1 transcripts bearing the Rev response element RNA target, and thereby inducing their nuclear egress. However, several groups have argued that Crm1 recruitment may not be sufficient for Rev function. Thus, several additional candidate cofactors for Rev have been proposed, and Rev has also been suggested to also inhibit the nuclear splicing of HIV-1 transcripts and/or to directly enhance their cytoplasmic translation. To examine whether Crm1 recruitment is, instead, sufficient to activate the nuclear export of viral mRNAs, we targeted a leucine-rich Crm1 binding domain, derived from a heterologous protein that normally plays no role in RNA metabolism, to HIV-1 RNAs and showed that this tethered Crm1 binding domain is sufficient to induce the nuclear export and cytoplasmic translation of late HIV-1 mRNA species. More importantly, we show that direct tethering of the Crm1 nuclear export factor to target mRNAs, by fusion to a heterologous RNA binding domain, is in and of itself sufficient to induce the nuclear export and cytoplasmic expression of the unspliced HIV-1 mRNAs that encode the viral Gag proteins.

HIV-1 Rev transactivator: a beta-subunit directed substrate and effector of protein kinase CK2

The phosphorylation of HIV-1 Rev by protein kinase CK2 is strictly dependent on the regulatory beta subunit of the kinase and is deeply affected by conformational changes of the substrate outside the phosphorylation site. Here we show that Rev modulates a variety of CK2 properties, including autophosphorylation, catalytic activity toward calmodulin, and susceptibility to polycationic effectors, whose common denominator is the involvement of the beta subunit. Rev's two major CK2 sites are located at its N-terminus, immediately adjacent to a helix-loop-helix motif. By comparing the behaviour of full-size Rev with that of synthetic peptides reproducing, with suitable modifications, its N-terminal 26 amino acids including the phosphoacceptor site (Ser 5, Ser 8) and amphipathic helix-1, it appears that the functional interaction of the N-terminal portion of Rev with the N-terminal domain of the beta subunit must rely on both electrostatic and hydrophobic interactions. The former mainly involve Rev's arginine-rich domain (residues 35-50) in helix-2, while the latter are mostly mediated by residues 12-24 of helix-1. These data disclose the possibility that, besides displaying protective, regulatory and targeting properties with respect to the catalytic subunit, the CK2 beta subunit also plays a role as a docking site for a subset of CK2 substrates.