Department of Microbiology, Immunology, and Tropical Medicine, The George Washington University, 2300 I St., NW, Ross Hall, Room 551, Washington, DC 20037, USA.
J Virol. 2010 May;84(9):4755-68. doi: 10.1128/JVI.00851-09. Epub 2010 Feb 17.
Human T-lymphotropic virus type 1 (HTLV-1) encodes the viral protein Tax, which is believed to act as a viral transactivator through its interactions with a variety of transcription factors, including CREB and NF-kappaB. As is the case for all retroviruses, the provirus is inserted into the host DNA, where nucleosomes are deposited to ensure efficient packaging. Nucleosomes act as roadblocks in transcription, making it difficult for RNA polymerase II (Pol II) to proceed toward the 3' end of the genome. Because of this, a variety of chromatin remodelers can act to modify nucleosomes, allowing for efficient transcription. While a number of covalent modifications are known to occur on histone tails in HTLV-1 infection (i.e., histone acetyltransferases [HATs], histone deacetylases [HDACs], and histone methyltransferases [HMTs]), evidence points to the use of chromatin remodelers that use energy from ATP hydrolysis to remodel nucleosomes. Here we confirm that BRG1, which is the core subunit of eight chromatin-remodeling complexes, is essential not only for Tax transactivation but also for viral replication. This is especially evident when wild-type infectious clones of HTLV-1 are used. BRG1 associates with Tax at the HTLV-1 long terminal repeat (LTR), and coexpression of BRG1 and Tax results in increased rates of transcription. The interaction of BRG1 with Tax additionally recruits the basal transcriptional machinery and removes some of the core histones from the nucleosome at the start site (Nuc 1). When using the BRG1-deficient cell lines SW13, C33A, and TSUPR1, we observed little viral transcription and no viral replication. Importantly, while these three cell lines do not express detectable levels of BRG1, much of the SWI/SNF complex remains assembled in the cells. Knockdown of BRG1 and associated SWI/SNF subunits suggests that the BRG1-utilizing SWI/SNF complex PBAF is responsible for HTLV-1 nucleosome remodeling. Finally, HTLV-1 infection of cell lines with a knockdown in BRG1 or the PBAF complex results in a significant reduction in viral production. Overall, we concluded that BRG1 is required for Tax transactivation and HTLV-1 viral production and that the PBAF complex appears to be responsible for nucleosome remodeling.
人类 T 淋巴细胞白血病病毒 1 型（HTLV-1）编码病毒蛋白 Tax，该蛋白被认为通过与各种转录因子（包括 CREB 和 NF-kappaB）相互作用而充当病毒转录激活因子。与所有逆转录病毒一样，前病毒插入宿主 DNA，在宿主 DNA 中沉积核小体以确保有效的包装。核小体在转录中充当障碍，使 RNA 聚合酶 II（Pol II）难以向基因组的 3' 端移动。正因为如此，多种染色质重塑因子可以作用于修饰核小体，从而实现有效的转录。尽管在 HTLV-1 感染中已知组蛋白尾部会发生多种共价修饰（即组蛋白乙酰转移酶 [HATs]、组蛋白去乙酰化酶 [HDACs] 和组蛋白甲基转移酶 [HMTs]），但有证据表明使用从 ATP 水解中获取能量的染色质重塑因子来重塑核小体。在这里，我们证实 BRG1 是八个染色质重塑复合物的核心亚基，不仅对 Tax 转录激活而且对病毒复制都是必不可少的。当使用 HTLV-1 的野生型感染性克隆时，这一点尤其明显。BRG1 与 HTLV-1 长末端重复序列（LTR）上的 Tax 结合，BRG1 和 Tax 的共表达导致转录率增加。BRG1 与 Tax 的相互作用还募集了基础转录机制，并从起始位点（Nuc 1）去除核小体中的一些核心组蛋白。当使用 BRG1 缺陷细胞系 SW13、C33A 和 TSUPR1 时，我们观察到很少有病毒转录，也没有病毒复制。重要的是，虽然这三个细胞系不表达可检测水平的 BRG1，但 SWI/SNF 复合物的大部分仍在细胞中组装。BRG1 和相关的 SWI/SNF 亚基的敲低表明，BRG1 利用的 SWI/SNF 复合物 PBAF 负责 HTLV-1 核小体重塑。最后，BRG1 或 PBAF 复合物敲低的细胞系中的 HTLV-1 感染导致病毒产量显著减少。总的来说，我们得出结论，BRG1 是 Tax 转录激活和 HTLV-1 病毒产生所必需的，并且 PBAF 复合物似乎负责核小体重塑。
