CVB3 RNA-dependent RNA polymerase 3D is ubiquitinated Some virus RNA-dependent RNA polymerases including the sindbis virus and the turnip yellow mosaic virus RNA polymerases have been demonstrated to be phosphorylated and ubiquitinated [31] . Although the role of ubiquitination of these RNA polymerases in the regulation of virus replication remains to be determined, such observation raises the interesting possibility that the ubiquitin/proteasome system may regulate CVB3 replication through ubiquitinating viral polymerase 3D, which is essential for initiating viral RNA replication. To examine whether coxsackieviral proteins are subjected to ubiquitination during viral infection, we performed immunoprecipitation with anti-ubiquitin antibody, followed by immunoblots using antibodies against 3D pol and viral capsid protein VP1, respectively. As shown in Fig. 5 , immunoreactive bands of around 60 kDa were detected in CVB3infected cells. Non-modified 3D pol has a molecular weight of about 53 kDa, thus this observation suggests that 3D pol likely undergoes post-translational modification by monoubiquitination. No protein ubiquitination was observed for VP1 (data not shown). Our results implicate that the ubiquitination process of CVB3 viral proteins might be required for successful replication of the virus.