Upon activation, NF-kappaB translocates into the nucleus and initiates biological events. This NF-kappaB signalling is mainly regulated by the protein kinase IKKbeta. Early in this signalling pathway, IKKbeta is phosphorylated for activation by several factors, such as pro-inflammatory cytokines and the Tax oncoprotein of HTLV-1. In cells infected by HTLV-1, IKKbeta is persistently phosphorylated and conjugated with monoubiquitin due to Tax expression. Although this Tax-induced monoubiquitination appears to be an important regulation system for IKKbeta, how the monoubiquitination occurs is unknown and its role in NF-kappaB signalling is still unclear. Here, we show that an E3-ubiquitin ligase Ro52 interacts weakly with wild-type IKKbeta but strongly with a phosphomimetic mutant IKKbeta to conjugate monoubiquitin in cooperation with an E2-ubiquitin-conjugating enzyme UbcH5B. These results suggest that the Tax-induced phosphorylation of IKKbeta causes an interaction with Ro52 for the subsequent monoubiquitination. NF-kappaB reporter assays have shown that the IKKbeta activity is suppressed by wild-type Ro52, but not by its inactive mutant. In addition, monoubiquitin fusion of IKKbeta reduced its activity for NF-kappaB signalling. We also found that Ro52 dramatically reduces the level of Tax. These results suggest that Ro52 down-regulates Tax-induced NF-kappaB signalling by monoubiquitinating IKKbeta and by reducing the level of Tax.