DCAF1 is required for the association of Vpr with DDB1. (A) A model of the Vpr-CRL4-DCAF1 E3 ubiquitin ligase complex in which Vpr directly contacts DCAF1. (B) HeLa cells were transfected with siRNAs targeting DCAF1 or DDB1. A control siRNA is included and two siRNAs were used against DCAF1. A day later, the cells were transfected again with HA-Vpr expression vector. After another two days, HA-Vpr complexes were immunoprecipitated with anti-HA antibody. Co-immunoprecipitated DCAF1 and DDB1 were detected on an immunoblot. A nonspecific band is indicated with an asterisk.