HIV-MEDIATED REPRESSION OF MHC CLASS I GENE EXPRESSION

MHC I类分子是病毒多肽的主要受体， 作为特定细胞毒性T淋巴细胞的靶点。HIV-1感染 T细胞株减少细胞表面I类分子和 使启动子活性最多降低12倍。压抑是 由HIV-1 Tat蛋白介导，来自剪接病毒 记录，确定了这个双外显子TAT的新活动， 与HIV LTR的反式激活不同，两者共同的是- 和两个外显子的纹身。TAT介导的压制的目标映射到 基本启动子。然而，Tat不直接与dna结合，也不会。 靶向TATAA或INR基本启动子元件。相反，它的 活性指向转录启动复合体。vbl.使用 酵母双杂交筛选，我们已经鉴定出一个转录因子 这与TAT相互作用，可能成为镇压的目标。研究 正在进一步描述这种相互作用，以及它的 在镇压中扮演的角色。Tat蛋白功能结构域的定位 证明了抑制和激活是由不同的 和可分离的域。TAT抑制物活性依赖于C-末端 序列，而反式激活依赖于N-末端序列；两者 函数需要核心序列。抑制物活动需要一个 包含由TAT的第二外显子编码的区域的结构域 基因，起始于73个氨基酸，C末端限制在氨基酸之间 酸80和83。TAT抑制因子的功能也取决于是否存在 位于蛋白质核心的第41位的赖氨酸。塔特 抑制物的活性完全不依赖于两个N-末端结构域 对激活是必需的，酸性片段和富含半胱氨酸 区域。相反，TAT的反式激活独立于第二种 TAT基因外显子编码区。作为对这一新模型的进一步支持 分离的TAT功能，我们已经证明在小鼠成纤维细胞中，TAT 抑制I类启动子活性，但不反式激活HIV Ltr.我们认为，不同的结构域在TAT的两个 在功能上不同的活动。

MHC class I molecules are the major receptors for viral peptides and serve as targets for specific cytotoxic T lymphocytes. HIV-1 infection of T cell lines decreases cell surface expression of class I and decreases the promoter activity by up to 12-fold. Repression is mediated by the HIV-1 Tat protein, derived from a spliced viral transcript, identifying a novel activity for this two-exon Tat, distinct from the transactivation of the HIV LTR common to both one- and two-exon Tat. The target of Tat-mediated repression maps to the basal promoter. However, Tat does not bind DNA directly, nor does it target either the TATAA or Inr basal promoter elements. Rather, its activity is directed to the transcription iniation complex. Using yeast-two hybrid screening, we have identified a transcription factor that interacts with Tat and may be the target for repression. Studies are in progress to further characterization this interaction, and its role in repression. Mapping of functional domains of Tat protein demonstrates that repression and activation are mediated by distinct and separable domains. Tat repressor activity depends on C-terminal sequences, whereas transactivation depends on N-terminal sequence; 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 completely independent of two N-terminal domains essential for transactivation, the acidic segment and the cystein 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 have shown that in murine fibroblasts, Tat represses class I promoter activity, but does not transactivate the HIV LTR. We propose that distinct structural domains mediate Tat's two functionally distinct activities.