HIV-MEDIATED REPRESSION OF MHC CLASS I GENE EXPRESSION

MHC I类分子是病毒肽的主要受体， 作为特异性细胞毒性T淋巴细胞的靶点。HIV-1感染 T细胞系的表达降低了I类细胞的细胞表面表达， 启动子活性降低了12倍。压抑是 由HIV-1达特蛋白介导，来自剪接的病毒 转录本，鉴定该双外显子达特的新活性， 与HIV LTR的反式激活不同， 和两个外显子的达特。Tat介导的抑制作用的靶点映射到 基础启动子然而，达特不直接结合DNA，也不 靶向TATAA或Inr基础启动子元件。相反，其 活性针对转录起始复合物。使用 通过酵母双杂交筛选，我们鉴定了一种转录因子 与达特相互作用，可能是抑制的目标。研究 正在进一步描述这种相互作用， 镇压的作用。达特蛋白功能域的定位 表明抑制和激活是由不同的 和可分离的域。达特阻遏物活性依赖于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。阻遏物的活性需要 包含由达特的第二外显子编码的区域的结构域 基因，从氨基酸73开始，氨基酸之间有C-末端限制， 酸80和83。达特阻遏物的功能也依赖于 位于蛋白质核心内的位置41处的赖氨酸。达特 阻遏物活性完全不依赖于两个N-末端结构域 反式激活所必需的酸性片段和富含半胱氨酸的 地区相反地，达特反式激活与第二个 达特外显子编码区。作为对这一新模式的进一步支持， 分离的达特功能，我们已经表明，在鼠成纤维细胞，达特， 抑制I类启动子活性，但不反式激活HIV LTR我们提出不同的结构域介导达特的两个 功能不同的活动。

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.