Then, medium was discarded and cells were washed twice with PBS

sical interaction, and indicates the same region of duck RIG-I is involved. The RIG-I SV, lacking exon 2 cannot interact with duck TRIM25. This region of human RIG-I includes the residue T55, which is critically involved in the interaction, as T55I or T55E mutants are inactive. Duck RIG-I does not have the residue T55, but has the conservative substitution alanine. The human RIG-I mutant T55A still interacts with human TRIM25, but is suboptimal compared to wild type. Finally, the presence of this splice variant in tissues at 3 dpi, when the response to infection is winding down, is consistent with the hypothesis that the splice variant functions as a dominant inhibitor of full length RIG-I to prevent MAVS activation, as demonstrated for the human RIG-I splice variant. However, we are unable to demonstrate inhibition of duck RIG-I signaling by the mutant SVRIG-I lacking exon 2, when both are equally expressed in chicken DF-1 cells, at amounts above the natural abundance of the transcript. Using mass spectrometry we demonstrated that TRIM25 attaches anchored ubiquitin to PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19650037 the duck RIG-I CARD domains at Orange Yellow S residues K167 and K193. In band 1, we saw evidence of K167 and K193 ubiquitination, and also evidence for unubiquitinated K167, consistent with monoubiquitin attached to either of these residues, as the size of this band would predict. In band 2, we saw ubiquitination at K193 only, and a strong signal for K63-linked polyubiquitin chains. Band 2 was always seen as a doublet, and the lower band disappeared when the ubiquitin lacking all lysine residues was used, precluding production of polyubiquitin chains. The size of the upper band 2 is consistent with ubiquitin attached at both K167 and K193, while the lower band is a K63-linked polyubiquitin chain attached at one residue. Finally, in the third band we detected only ubiquitin attached at K193, and the size of this band correlated with the size of a chain of three ubiquitins. The pattern of ubiquitinated bands appeared identical to the bands arising from ubiquitination of human RIG-I, despite the ubiquitination of different residues. We created a double mutant K167R/K193R form of the RIG-I card domains that could not be ubiquitinated by human or duck TRIM25. Independent mutation of each site did not disrupt the attachment of ubiquitin, suggesting that attachment can occur at either of the identified sites. K167 lies close to the predicted site of phosphorylation at S168 in duck. Dephosphorylation of S168 may be a necessary step for the ubiquitination at either K167 or K193, and ubiquitination may prevent re-phosphorylation. No other lysines were found modified, and it appears that no other sites can be modified, since mutation of both lysines abrogates attachment of any ubiquitin. We also saw no attachment of ubiquitin at sites typically modified by RIPLET, as expected given the lack of RIPLET in the chicken genome. The double mutant form of RIG-I, that cannot be ubiquitinated, indirectly demonstrates that activation of the duck CARD domains by duck or human TRIM25 does not require any anchored ubiquitin chains. TRIM25 produces unanchored ubiquitin chains that activate RIG-I in vitro, but the importance of this has not been demonstrated in vivo. The double mutant of duck RIG-I CARD domains, still interacts with human or duck TRIM25 and can be activated. The most likely explanation is that duck RIG-I CARD domains are associating with unanchored polyubiquitin produced by TRIM25. This is in di