Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, Canada.
PLoS One. 2012;7(12):e53036. doi: 10.1371/journal.pone.0053036. Epub 2012 Dec 31.
In vertebrates, poly(A) binding protein (PABP) is known to exist in five different isoforms. PABPs are primarily cytosolic with the exception of the nuclear PABP (PABPN1), which is located in the nucleus. Within the nucleus, PABPN1 is believed to bind to the poly(A) tail of nascent mRNA and along with cleavage and polyadenylation specificity factor (CPSF) define the length of the newly synthesized poly(A) tail.
METHODOLOGY/PRINCIPAL FINDINGS: The cellular role of PABP1 has been extensively studied over the years; however, the function of other PABPs remains poorly defined. In order to understand the role of PABPN1 in cellular mRNA metabolism and it's interrelation with other PABPs, we depleted PABPN1 using RNAi in HeLa and HEK293 cells. Our results show that PABPN1 depletion did not have any effect on the poly(A) tail length, nuclear export of mRNA, mRNA translation, and transcription. Rather, PABPN1 depletion resulted in a compensatory response as observed by increased level of PABP5 and nuclear accumulation of PABP4. In addition, PABP4 was associated with the poly(A) tract of pre-mRNA and CPSF in PABPN1 depleted cells. Nevertheless, PABPN1 depletion significantly affected cell survival as evidenced by an increase in apoptosis markers: phosphorylated p53 and PUMA and as judged by the expression of ER stress marker GRP78.
Our results suggest that although function of PABPN1 may be compensated by nuclear translocation of PABP4 and perhaps by increase in the cytoplasmic abundance of PABP5, these were not sufficient to prevent apoptosis of cells. Thus PABPN1 may have a novel anti apoptotic role in mammalian cells.
在脊椎动物中，poly(A) 结合蛋白 (PABP) 已知存在五种不同的同工型。PABPs 主要存在于细胞质中，核 PABP (PABPN1) 除外，它位于细胞核中。在细胞核内，PABPN1 被认为与新生 mRNA 的 poly(A) 尾巴结合，并与切割和多聚腺苷酸化特异性因子 (CPSF) 一起定义新合成的 poly(A) 尾巴的长度。
方法/主要发现：多年来，人们对 PABP1 的细胞作用进行了广泛研究；然而，其他 PABPs 的功能仍未得到明确界定。为了了解 PABPN1 在细胞 mRNA 代谢中的作用及其与其他 PABPs 的相互关系，我们使用 RNAi 在 HeLa 和 HEK293 细胞中耗尽 PABPN1。我们的结果表明，PABPN1 耗尽对 poly(A) 尾巴长度、mRNA 的核输出、mRNA 翻译和转录没有任何影响。相反，PABPN1 耗尽导致代偿性反应，如 PABP5 水平升高和 PABP4 核积累。此外，PABP4 与 PABPN1 耗尽细胞中 pre-mRNA 的 poly(A) 区和 CPSF 相关。尽管如此，PABPN1 耗尽显著影响细胞存活，这表现在凋亡标志物：磷酸化 p53 和 PUMA 的增加以及内质网应激标志物 GRP78 的表达。
我们的结果表明，尽管 PABPN1 的功能可能通过 PABP4 的核易位和细胞质中 PABP5 丰度的增加得到补偿，但这不足以防止细胞凋亡。因此，PABPN1 可能在哺乳动物细胞中具有新的抗凋亡作用。
