Our previous work demonstrated that following human cytomegalovirus (HCMV) infection of fibroblasts, there was a protein-protein interaction between the HCMV IE1-72 immediate-early (IE) protein and the cellular p107 protein which resulted in the alleviation of p107-mediated transcriptional repression of E2F-responsive promoters. In a further characterization of this interaction, we now show that IE1-72 binds to the N-terminal portion of p107, not the C-terminal 'pocket' region that binds E2F-4, and where a number of other viral gene products bind. Additionally, we show that exons 2 and 3 of IE1-72 are required for binding to p107. After mapping the binding domains, we next wanted to address the additional functional consequences of this interaction. It is well known that p107 can negatively regulate cell growth. To examine whether IE1-72 can also overcome this growth suppression, we transfected and infected or cotransfected various constructs into SAOS-2 cells. We showed that infection of SAOS-2 cells was capable of alleviating p107-mediated growth suppression. Additionally, we showed that IE1-72 alone is capable of overcoming p107-mediated growth arrest. Alleviation of this repression by IE1-72 is dependent on the protein-protein interaction between p107 and IE1-72 as deletion mutants of either protein which lack the identified binding domains fail to achieve this effect. These data indicate that the IE1-72 protein is capable of overcoming p107-mediated blocks in cellular proliferation, events that occur in both productive and non-productive HCMV infections.
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