Srinivasan A, McClellan A J, Vartikar J, Marks I, Cantalupo P, Li Y, Whyte P, Rundell K, Brodsky J L, Pipas J M
Department of Biological Sciences, University of Pittsburgh, Pennsylvania 15260, USA.
Mol Cell Biol. 1997 Aug;17(8):4761-73. doi: 10.1128/MCB.17.8.4761.
Simian virus 40 (SV40) encodes two proteins, large T antigen and small t antigen that contribute to virus-induced tumorigenesis. Both proteins act by targeting key cellular regulatory proteins and altering their function. Known targets of the 708-amino-acid large T antigen include the three members of the retinoblastoma protein family (pRb, p107, and p130), members of the CBP family of transcriptional adapter proteins (cap-binding protein [CBP], p300, and p400), and the tumor suppressor p53. Small t antigen alters the activity of phosphatase pp2A and transactivates the cyclin A promoter. The first 82 amino acids of large T antigen and small t antigen are identical, and genetic experiments suggest that an additional target(s) important for transformation interacts with these sequences. This region contains a motif similar to the J domain, a conserved sequence found in the DnaJ family of molecular chaperones. We show here that mutations within the J domain abrogate the ability of large T antigen to transform mammalian cells. To examine whether a purified 136-amino-acid fragment from the T antigen amino terminus acts as a DnaJ-like chaperone, we investigated whether this fragment stimulates the ATPase activity of two hsc70s and discovered that ATP hydrolysis is stimulated four- to ninefold. In addition, ATPase-defective mutants of full-length T antigen, as well as wild-type small t antigen, stimulated the ATPase activity of hsc70. T antigen derivatives were also able to release an unfolded polypeptide substrate from an hsc70, an activity common to DnaJ chaperones. Because the J domain of T antigen plays essential roles in viral DNA replication, transcriptional control, virion assembly, and tumorigenesis, we conclude that this region may chaperone the rearrangement of multiprotein complexes.
猴病毒40（SV40）编码两种蛋白质，大T抗原和小t抗原，它们与病毒诱导的肿瘤发生有关。这两种蛋白质都通过靶向关键的细胞调节蛋白并改变其功能来发挥作用。已知的708个氨基酸的大T抗原的靶标包括视网膜母细胞瘤蛋白家族的三个成员（pRb、p107和p130）、转录衔接蛋白CBP家族的成员（帽结合蛋白[CBP]、p300和p400）以及肿瘤抑制因子p53。小t抗原改变磷酸酶pp2A的活性并反式激活细胞周期蛋白A启动子。大T抗原和小t抗原的前82个氨基酸是相同的，遗传实验表明，对转化很重要的其他靶标与这些序列相互作用。该区域包含一个类似于J结构域的基序，这是在分子伴侣的DnaJ家族中发现的保守序列。我们在此表明，J结构域内的突变消除了大T抗原转化哺乳动物细胞的能力。为了检查从T抗原氨基末端纯化的136个氨基酸的片段是否作为类似DnaJ的伴侣蛋白起作用，我们研究了该片段是否刺激两种热休克蛋白70（hsc70）的ATP酶活性，并发现ATP水解被刺激了4至9倍。此外，全长T抗原的ATP酶缺陷突变体以及野生型小t抗原刺激了hsc70的ATP酶活性。T抗原衍生物也能够从hsc70释放未折叠的多肽底物，这是DnaJ伴侣蛋白共有的活性。由于T抗原的J结构域在病毒DNA复制、转录控制、病毒体组装和肿瘤发生中起重要作用，我们得出结论，该区域可能辅助多蛋白复合物的重排。
