Wang Jun, Vock Vita M, Li Shulin, Olivas O Renee, Wilkinson Miles F
Department of Immunology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, USA.
J Biol Chem. 2002 May 24;277(21):18489-93. doi: 10.1074/jbc.M111781200. Epub 2002 Mar 11.
Nonsense-mediated decay (NMD) is an RNA surveillance pathway that degrades mRNAs containing premature termination codons (PTC). T-cell receptor (TCR) and immunoglobulin (Ig) transcripts, which are encoded by genes that very frequently acquire PTCs during lymphoid ontogeny, are down-regulated much more dramatically in response to PTCs than are other known transcripts. Another feature unique to TCR, Ig, and a subset of other mRNAs is that they are down-regulated in response to nonsense codons in the nuclear fraction of cells. This is paradoxical, as the only well recognized entity that recognizes nonsense codons is the cytoplasmic translation apparatus. Therefore, we investigated whether translation is responsible for this nuclear-associated mechanism. We found that the down-regulation of TCR-beta transcripts in response to nonsense codons requires several features of translation, including an initiator ATG and the ability to scan. We also found that optimal down-regulation depends on a Kozak consensus sequence surrounding the initiator ATG and that it can be initiated by an internal ribosome entry site, neither of which has been demonstrated before for any other PTC-bearing mRNA. At least a portion of this down-regulatory response is mediated by the NMD pathway as antisense hUPF2 transcripts increased the levels of PTC-bearing TCR-beta transcripts in the nuclear fraction of cells. We conclude that a hUPF2-dependent RNA surveillance pathway with translation-like features operating in the nuclear fraction of cells prevents the expression of potentially deleterious truncated proteins encoded by non-productively rearranged TCR genes.
无义介导的衰变（NMD）是一种RNA监测途径，可降解含有提前终止密码子（PTC）的mRNA。T细胞受体（TCR）和免疫球蛋白（Ig）转录本由在淋巴细胞发育过程中经常获得PTC的基因编码，与其他已知转录本相比，它们对PTC的反应下调更为显著。TCR、Ig和其他一些mRNA子集的另一个独特特征是，它们在细胞的核部分中对无义密码子有反应而下调。这是自相矛盾的，因为唯一被广泛认可的识别无义密码子的实体是细胞质翻译装置。因此，我们研究了翻译是否对此核相关机制负责。我们发现，TCR-β转录本对无义密码子的下调需要翻译的几个特征，包括起始密码子ATG和扫描能力。我们还发现，最佳下调依赖于起始密码子ATG周围的科扎克共有序列，并且它可以由内部核糖体进入位点启动，这两个特征此前在任何其他含有PTC的mRNA中均未得到证实。至少部分这种下调反应是由NMD途径介导的，因为反义hUPF2转录本增加了细胞核对含有PTC的TCR-β转录本的水平。我们得出结论，一种具有翻译样特征、在细胞核部分起作用的hUPF2依赖性RNA监测途径可阻止由非生产性重排的TCR基因编码的潜在有害截短蛋白的表达。
