Cyclin K/CPR4 inhibits primate lentiviral replication by inactivating Tat/positive transcription elongation factor b-dependent long terminal repeat transcription

Novel role of HSP40/DNAJ in the regulation of HIV-1 replication

OBJECTIVES

DNAJ/HSP40 is an evolutionarily conserved family of proteins bearing various functions. Historically, it has been emphasized that HSP40/DNAJ family proteins play a positive role in infection of various viruses. We identified DNAJ/HSP40B6 as a potential negative regulator of HIV-1 replication in our genetic screens. In this study, we investigated the functional interactions between HIV-1 and HSP40 family members.

DESIGN

We took genetic and comparative virology approaches to expand the primary observation.

METHODS

Multiple HSP40/DNAJ proteins were tested for their ability to inhibit replication of adenovirus, herpes simplex virus type 1, HIV-1, and vaccinia virus. The mechanism of inhibition was investigated by using HSP40/DNAJ mutants and measuring the efficiencies of each viral replication steps.

RESULTS

HSP40A1, B1, B6, and C5, but not C3, were found to be able to limit HIV-1 production. This effect was specific to HIV-1 for such effects were not detected in adenovirus, herpes simplex virus type 1, and vaccinia virus. Genetic analyses suggested that the conserved DNAJ domain was responsible for the inhibition of HIV-1 production through which HSP40 regulates HSP70 ATPase activity. Interestingly, HSP40s lowered the levels of steady-state viral messenger RNA. This was not attributed to the inhibition of Tat/long terminal repeat-driven transcription but the downregulation of Rev expression.

CONCLUSIONS

This is the first report providing evidence that HSP70-HSP40 complex confers an innate resistance specific to HIV-1. For their interferon-inducible nature, HSP40 family members should account for the anti-HIV-1 function of interferon.

Regulation of cyclin T1 expression and function by an alternative splice variant that skips exon 7 and contains a premature termination codon

Cyclin T1 (CCNT1), a gene containing nine exons, forms the positive transcription elongation factor b (P-TEFb) complex and regulates a wide variety of biological processes including transcription. We discovered a novel splice variant of CCNT1 that lacks exon 7 (dE7). RT-PCR analysis revealed that the dE7 transcript was detected in almost all tissues examined. The dE7/FL transcript ratio was high in quiescent peripheral blood mononuclear cells (PBMC) and in tissues poor in cell division; however, it was low in activated PBMC and in tissues with high cell proliferative potential. These results suggest that exon 7 skipping is linked to cell cycle progression. Increasing the dE7/FL transcript ratio resulted in the reduction of CCNT1 protein levels, indicating that the expression of CCNT1 protein is controlled by exon skipping. Exon 7 skipping yields a +1 frameshift at exon 8, which generates a premature termination codon (PTC). The dE7 transcript levels increased when cells were treated with the protein synthesis inhibitor cycloheximide (CHX) or a kinase inhibitor wortmannin (WORT), whilst the FL transcript levels were unchanged, suggesting that the dE7 transcript is a target of nonsense-mediated decay (NMD). Importantly, reduction of dE7 transcript by WORT correlated well with the decrement of CCNT1 protein expression. The dE7 transcript would produce an approximately 23kDa protein that covers approximately 70% of the cyclin box. The ectopically expressed dE7 protein physically interacted with CDK9 and competed with FL CCNT1 for CDK9, thus should act dominant-negatively on FL CCNT1. The replication of human immunodeficiency virus type 1 (HIV-1), heavily dependent on the CCNT1 function, was inhibited by dE7 protein through the attenuation of Tat/long terminal repeat (LTR)-driven transcription. Taken together, these results suggest that dE7 is a novel splice variant that regulates the expression and function of CCNT1.

Cyclin K inhibits HIV-1 gene expression and replication by interfering with cyclin-dependent kinase 9 (CDK9)-cyclin T1 interaction in Nef-dependent manner

Human immunodeficiency virus-1 (HIV-1) exploits a number of host cellular factors for successful survival and propagation. The viral protein Nef plays an important role in HIV-1 pathogenesis by interacting with various cellular proteins. In the present work, we identified Cyclin K (CycK) as a novel Nef-interacting protein, and for the first time, we showed that CycK inhibits HIV-1 gene expression and replication in a Nef-dependent manner. The positive elongation factor b complex comprising cyclin-dependent kinase 9 (CDK9) and Cyclin T1 is a critical cellular complex required for viral gene expression and replication. Enhanced expression of CycK in the presence of Nef induced CycK-CDK9 binding, which prevented CDK9-Cyclin T1 complex formation and nuclear translocation of CDK9, resulting in inhibition of HIV-1 long terminal repeat-driven gene expression. Furthermore, this effect of CycK was not observed with Nef-deleted virus, indicating the importance of Nef in this phenomenon. Finally, silencing of CycK in HIV-1-infected cells resulted in increased translocation of CDK9 into the nucleus, leading to increased viral gene expression and replication. These data also suggest that endogenous CycK might act as an inhibitory factor for HIV-1 gene expression and replication in T-cells. Thus, our results clearly demonstrate that CycK utilizes HIV-1 Nef protein to displace CycT1 from the positive elongation factor b complex, resulting in inhibition of HIV-1 gene expression and replication.

Cell-type-specific proteome and interactome: using HIV-1 Tat as a test case

HIV-1 is a small retrovirus that wreaks havoc on the human immune system. It is a puzzle to the scientific community how a virus that encodes only nine proteins can take complete control of its host and redirect the cell to complete replication or maintain latency when necessary. One way to explain the control elicited by HIV-1 is through numerous protein partners that exist between viral and host proteins, allowing HIV-1 to be intimately involved in virtually every aspect of cellular biology. In addition, we postulate that the complexity exerted by HIV-1 can not merely be explained by the large number of protein-protein interactions documented in the literature but, rather, cell-type-specific interactions and post-translational modifications of viral proteins must be taken into account. We use HIV-1 Tat and its influence on viral transcription as an example of cell-type-specific complexity. The influence of post-translational modifications (acetylation and methylation), as well as subcellular localization on Tat binding partners, is also discussed.

Identification of the P-TEFb complex-interacting domain of Brd4 as an inhibitor of HIV-1 replication by functional cDNA library screening in MT-4 cells

We conducted a phenotypic cDNA screening using a T cell line-based assay to identify human genes that render cells resistant to human immunodeficiency virus type 1 (HIV-1). We isolated potential HIV-1 resistance genes, including the carboxy terminal domain (CTD) of bromodomain-containing protein 4 (Brd4). Expression of GFP-Brd4-CTD was tolerated in MT-4 and Jurkat cells in which HIV-1 replication was markedly inhibited. We provide direct experimental data demonstrating that Brd4-CTD serves as a specific inhibitor of HIV-1 replication in T cells. Our method is a powerful tool for the identification of host factors that regulate HIV-1 replication in T cells.