title: HIV Tat protein requirements for transactivation and repression of transcription are separable.
abstract: The HIV Tat protein, primarily characterized as a transcriptional activator of the viral long terminal repeat (LTR), is also a potent repressor of major histocompatibility complex (MHC) class I transcription. In the present study, we demonstrate that these two functional activities are distinct and mediated by discrete, but overlapping, structural domains of Tat. Tat repressor activity depends on C-terminal sequences, whereas transactivation depends on N-terminal sequences; both functions require core sequences. The repressor activity requires a domain encompassing the region encoded by the second exon of the Tat gene, beginning at amino acid 73, with a C-terminal limit between amino acids 80 and 83. Tat repressor function also depends on the presence of a lysine at position 41, located within the core of the protein. Tat repressor activity is independent of two N-terminal domains essential for transactivation: the acidic segment and the cysteine-rich region. Conversely, Tat transactivation is independent of the second exon-encoded region of Tat. As further support for this novel model of separable Tat functions, we show that in murine fibroblasts, Tat represses class I promoter activity, but does not transactivate the HIV LTR. We propose that distinct structural domains mediate the two functionally distinct activities associated with the Tat protein.
authors.affiliation: Experimental Immunology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Brown
authors.forename: T K
authors.initials: TK
authors.lastname: Howcroft
authors.forename: D S
authors.initials: DS
authors.lastname: Singer
mesh_terms: D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D005091:Exons; D005786:Gene Expression Regulation; D015696:Gene Products, tat / Q000235:genetics / Q000378:metabolism*; D006678:HIV / Q000235:genetics*; D016325:HIV Long Terminal Repeat; D006367:HeLa Cells; D015395:Histocompatibility Antigens Class I / Q000096:biosynthesis; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D014162:Transfection; D054322:tat Gene Products, Human Immunodeficiency Virus
chemical_list: D015696:Gene Products, tat; D015395:Histocompatibility Antigens Class I; D010446:Peptide Fragments; D011994:Recombinant Proteins; D012097:Repressor Proteins; D054322:tat Gene Products, Human Immunodeficiency Virus