Cyclin K inhibits HIV-1 gene expression and replication by interfering with 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 have identified Cyclin K (CycK) as a novel Nefinteracting protein and for the first time we show that CycK inhibits HIV-1 gene expression and replication in Nef-dependent manner. The positive elongation factor b complex (P-TEFb) comprising of 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 induces CycK-CDK9 binding, which prevents 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 result in increased translocation of CDK9 in to the nucleus leading to increased viral gene expression and replication. This data also suggests 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 P-TEFb complex resulting in inhibition of HIV-1 gene expression and replication.