Overexpression of Sam68 functionally substitutes for, as well as synergizes with, human immunodeficiency virus type 1 (HIV-1) Rev in RRE (Rev response element)-mediated gene expression and virus replication. In addition, COOH-terminal deletion and/or point mutants of Sam68 exhibit a transdominant negative phenotype for HIV replication. Sam68 is a member of KH domain family that includes SLM-1, SLM-2 (Sam68 like mammalian); and QKI-5, QKI-6, and QKI-7 (mouse quaking) proteins. The objective of this study was to examine the effects of these KH family proteins on RRE- and CTE (constitutive transport element of type-D retrovirus)-mediated transactivation. We now report that SLM-1 and SLM-2 proteins, which are the closest relatives of Sam68, marginally enhanced RRE-mediated transactivation, while QK isoforms that are distant relatives of Sam68 had no effect. Interestingly, these proteins still enhanced the effect of Rev in RRE-mediated gene expression. The increase in chloramphenicol acetyltransferase activity was also reflected at the levels of cytoplasmic RRE-chloramphenicol acetyltransferase mRNAs, indicating that Sam68 and KH proteins may have been involved in the stability or export of unspliced RNA. The increase in Rev activity was sensitive to leptomycin B, but not to olomoucine, indicating that the effect of SLM-1, SLM-2, QKI-5, QKI-6, and QKI-7 is exerted through a CRM-1-dependent mRNA export pathway. Thus, KH family proteins play an important role in the post-transcriptional regulation of HIV.
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