The Herpes Simplex Virus 1 Us11 Protein Cooperates with Suboptimal Amounts of Human Immunodeficiency Virus Type 1 (HIV-1) Rev Protein to Rescue HIV-1 Production

Abstract The human immunodeficiency virus type 1 (HIV-1) RNA-binding Rev protein governs the expression of structural and enzymatic viral proteins at a posttranscriptional level. Binding of Rev to the stem-loop IIB (SLIIB) sequence of the Rev-response element (RRE) within unspliced and singly spliced viral mRNAs and to the nuclear export signal-binding receptor, hCRM1 (or exportin 1), is required for the export of these transcripts to the cytoplasm. We have previously shown that herpes simplex virus type 1 (HSV-1) RNA-binding Us11 protein is able to bind the RRE and substitute for Rev in inducing the expression of HIV-1 envelope glycoproteins. We show here that Us11 cannot substitute for Rev in rescuing a rev -deleted HIV-1 provirus. However, HIV-1 production is observed when Us11 is expressed with suboptimal amounts of Rev. An in vivo RNA–protein binding assay indicates that Us11 is unable to directly interact with the SLIIB RNA but can bind Rev assembled on that stem–loop structure. This association of US11 with Rev, which was confirmed by in vivo coimmunoprecipitation and GST-pulldown assays, therefore underlies a biological Us11–Rev cooperation. Furthermore this cooperation was shown to remain susceptible to the effect of leptomycin B, which blocks the binding of hCRM1 to the nuclear export signal of Rev. These observations performed with intron-containing constructs provide evidence that HSV-1 Us11 protein is not directly involved in the cytoplasmic accumulation of viral mRNAs but may be rather acting as an auxiliary protein, thus allowing this retroviral protein to fulfill the nuclear export of these transcripts and to rescue HIV-1 production.