Cyclin box 1 domain of Cyclin K is necessary for its inhibitory activity on LTR-driven gene expression and HIV-1 virus production. A, schematic representation of Cyclin K mutants generated and used in the present study. B, cyclin box 1 domain is required for the inhibition of HIV-1 LTR-driven luciferase activity. HEK-293T cells were transfected with LTR-Luc reporter and pNL4-3 molecular clone along with various CycK mutant expression vectors as indicated in the figure. 24 h post-transfection, cells were harvested followed by a luciferase assay of the lysate. C, cyclin box 1 domain is required for the inhibition of HIV-1 virus production. HEK-293T cells were transfected with LTR-Luc reporter and pNL4-3 molecular clone along with various CycK mutant expression vectors as indicated in the figure. 24 h post-transfection, cell supernatants were used to analyze the virus production by a p24 antigen capture ELISA. D, expression of different Cyclin K mutant proteins in transfected HEK-293T cells. HEK-293T cell lysates were examined for the expression of various CycK mutants using Xpress epitope antibody and GAPDH by Western blot analysis in the experiments in B and C.