The HIV-1 vif gene is essential for infectious virus production by nonpermissive cells (NP), which include CD4-positive T-lymphocytes and macrophages and some leukemic T-cell lines, but is irrelevant in other cell lines that are termed permissive (P). NP cells can be infected with vif-deleted HIV-1 derived from P cells, but the infected NP cells release only noninfectious virions. These noninfectious virions cannot be rescued by vif expression in target cells, and they are believed to have a defect that reduces efficiency of reverse transcription. We and another laboratory recently showed the dominance of the NP phenotype in PxNP heterokaryons, suggesting that NP cells contain a factor that can inactivate HIV-1 and that this factor is counteracted by Vif. Using a human lymphocyte cDNA library in a yeast two-hybrid screen with Vif as bait, we have now identified the interferon-inducible protein Sp140 as an excellent candidate for this NP factor. Briefly, Sp140 occurs in all tested NP cells but not in P cells. Sp140 occurs in nuclear bodies (NBs) that contain several proteins covalently modified by addition of the ubiquitin-related protein Sumo-1. Expression of Sp140 in Hela-CD4 cells induces sumoylation of at least one protein, and coexpression of Vif blocks this induction. HIV-1 infections induce Sp140 emigration to the cytosol and its partial colocalization with Vif. We propose: (i) Verify Vif interaction with Sp140 and with other potential partners by genetic, immunological, and biochemical methods. (ii) Analyze Sp140 isoforms and other potential Vif partners for their abilities to inactivate vif-deleted HIV-1. (iii) Identify active sites for interaction of Vif with Sp140 and with other potential partners. (iv) By two-dimensional electrophoresis, identify sumoylated and non-sumoylated proteins induced in P cells by Sp140, and determine how Vif influences these inductions. Similarly, determine whether Vif expression in NP cells alters protein sumoylation and whether vif-deleted HIV-1 made in P or NP cells contains any sumoylated protein. (iv) Interestingly, the herpes simplex virus type 1 ICPO protein targets NBs and specifically causes the elimination of some sumoylated NB proteins, and proteins encoded by CMV, EBV, adenoviruses and arenaviruses appear to have similar activities. Determine whether expression of these other viral proteins converts NP cells to P. This work may unveil a novel target for drug development in AIDS.