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。压抑是居中的 通过HIV-1Tat蛋白，来自拼接的病毒转录本， 确定该双外显子TAT的一个新活性，与 单外显子TAT和双外显子TAT共同的HIV LTR反式激活。这个 TAT介导的抑制作用的靶标定位于基本启动子。然而， TAT不直接结合DNA，也不针对TATAA或 INR基本启动子元件。相反，它的活动是针对 转录启动复合体。使用酵母双杂交筛选，我们 已经确定了一种与TAT相互作用的转录因子，并可能 成为镇压的目标。研究正在进行中，以进一步 描述这种相互作用及其在镇压中的作用。在进在进 初步的体外分析，我们已经验证了相互作用 具有TAT的TFIID组件。此外，MHC的体外转录 I类启动子受TAT抑制。 TAT蛋白功能结构域的图谱显示 抑制和激活是由不同的和可分离的 域名。TAT抑制物的活性取决于C-末端序列，而 激活依赖于N-末端序列；这两种功能都需要 核心序列。抑制物活动需要一个包含以下内容的域 由TAT基因的第二外显子编码的区域，从 氨基酸73，C-末端限制在80-83个氨基酸之间。 TAT抑制物的功能也依赖于赖氨酸的存在 第41位，位于蛋白质核心内。TAT抑制子 活性完全独立于两个N-末端结构域 对于反式激活，酸性部分和富含半胱氨酸的区域。 相反，TAT反式激活与第二外显子无关 TAT的编码区。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 characterize this interaction, and its role in repression. In in preliminary in vitro analysis, we have verified the interaction of the TFIID component with Tat. Furthermore, in vitro transcription of the MHC class I promoter is repressed by Tat. 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.