The V protein of simian virus 5 (SV5) facilitates the ubiquitination and subsequent proteasome-mediated degradation of STAT1. Here we show, by visualizing direct protein-protein interactions and by using the yeast two-hybrid system, that while the SV5 V protein fails to bind to STAT1 directly, it binds directly and independently to both DDB1 and STAT2, two cellular proteins known to be essential for SV5-mediated degradation of STAT1. We also demonstrate that STAT1 and STAT2 interact independently of SV5 V and show that SV5 V protein acts as an adaptor molecule linking DDB1 to STAT2/STAT1 heterodimers, which in the presence of additional accessory cellular proteins, including Cullin 4a, can ubiquitinate STAT1. Additionally, we show that the avidity of STAT2 for V is relatively weak but is significantly enhanced by the presence of both STAT1 and DDB1, i.e., the complex of STAT1, STAT2, DDB1, and SV5 V is more stable than a complex of STAT2 and V. From these studies we propose a dynamic model in which SV5 V acts as a bridge, bringing together a DDB1/Cullin 4a-containing ubiquitin ligase complex and STAT1/STAT2 heterodimers, which leads to the degradation of STAT1. The loss of STAT1 results in a decrease in affinity of binding of STAT2 for V such that STAT2 either dissociates from V or is displaced from V by STAT1/STAT2 complexes, thereby ensuring the cycling of the DDB1 and SV5 V containing E3 complex for continued rounds of STAT1 ubiquitination and degradation.Simian virus type 5 (SV5) is classified within the genus Rubulavirus of the subfamily Paramyxovirinae of the family Paramyxoviridae (18). It is now well established that most members of the Paramyxovirinae subfamily at least partially circumvent the interferon (IFN) response by blocking IFN signaling and reducing the production of IFN by infected cells (for reviews see (1,10,13,22,32). In human cells, SV5 blocks both IFN-␣/␤ and IFN-␥ signaling by targeting STAT1 (a transcription factor which is essential for IFN signaling) for proteasome-mediated degradation (3,7,23,25,36,37). The molecular mechanisms by which SV5 targets STAT1 for degradation have been the subject of several recent investigations, and of the virus proteins, only the V protein is required to mediate this process (3,7,26).The SV5 V protein is the 222-amino-acid product of a faithful mRNA copy of the second open reading frame (the V/P gene) of the virus genome. V has been shown to be a multifunctional protein that, apart from its involvement in STAT1 degradation, also interacts with an IFN-inducible DExD/H box helicase mda-5 to limit the production of IFN (1), binds singlestranded RNA (20) and may act as a chaperone keeping the nucleoprotein of the virus soluble (28). STAT1 degradation, mediated by SV5 V protein, is independent of IFN signaling or phosphorylation of STAT proteins (3, 24). However, there is an absolute requirement for STAT2 in STAT1 degradation and consequently, SV5 infection fails to induce the degradation of STAT1 in STAT2-deficient (U6A) cells (26) or in 2fTGH cells that ex...

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