Xbp1的ligase发现

两篇文章，都是2014年发表，第二篇的筛选更有趣一点：

The mammalian tRNA ligase complex mediates splicing of XBP1 mRNA and controls antibody secretion in plasma cells | The EMBO Journal

A Synthetic Biology Approach Identifies the Mammalian UPR RNA Ligase RtcB: Molecular Cell

第一篇里有RTCB在Hela cell的定位（both nucleus and cytosol）

第二篇文章提到，RTCB对于XBP1的非经典剪接可能还有additional pathway.

In RtcB cKO cells, a small portion of XBP1s was still produced in the absence of RtcB, suggesting that an additional RNA ligase might compensate for RtcB function. Unlike mammalian cells, yeast HAC1 mRNA splicing is mediated by the 5′-PO₄/3′-OH ligation pathway utilizing tRNA ligase Trl1p and 2′-phosphotransferase Tpt1p (Gonzalez et al., 1999; Schwer et al., 2004; Sidrauski et al., 1996). Consistent with this observation, a similar RNA ligase activity has been detected biochemically in HeLa cell extract, but the identity of such ligase remains unknown (Zillmann et al., 1991). Therefore, it is possible that an RNA ligase in the 5′-PO₄/3′-OH pathway compensates for loss of RtcB ligase activity in RtcB-null cells. Nevertheless, we have shown that the cells expressing ligase-dead RtcB exhibited a nearly complete loss of XBP1s protein and XBP1s mRNA, suggesting that RtcB^C122A may act as a dominant-negative protein to prevent XBP1u substrates from being ligated by a compensatory RNA ligase.