7SL RNA interaction is insufficient for incorporation of SRP54 protein into HIV-1 particles. (A) Cell lysates of untransfected HeLa cells were immunoprecipitated with an SRP54-specific antibody (IP) or were mock-precipitated (Ctrl). Aliquots of total cell lysate (Total) and immunoprecipitates were subjected to immunoblot analysis using antibodies to SRP54 (α-SRP54), α-tubulin (α-tubulin). RNA was extracted from remaining cell lysate and immunoprecipitates and used for RT-PCR amplification of 7SL RNA. (B) HeLa cells were transfected vif-defective variants of either pNL4-3 (43ΔVif), pC-Help (C-HelpΔVif, or mS.1 (mS.1ΔVif). Transfected cells and virus-containing supernatants were harvested 24 h after transfection. Virus-containing supernatants were purified and concentrated as described in Methods. Cell and viral lysates were analyzed by immunoblotting for virus production using an HIV-positive patient serum (APS). Expression and packaging of SRP54 was analyzed using an SRP54-specific antibody oα-SRP54). Total cellular RNA and RNA extracted from concentrated viruses was used for RT-PCR amplification of 7SL RNA.