Tirasophon W, Lee K, Callaghan B, Welihinda A, Kaufman R J
Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan 48109-0650, USA.
Genes Dev. 2000 Nov 1;14(21):2725-36. doi: 10.1101/gad.839400.
The unfolded protein response (UPR) is a signal transduction pathway that is activated by the accumulation of unfolded proteins in the endoplasmic reticulum (ER). In Saccharomyces cerevisiae the ER transmembrane receptor, Ire1p, transmits the signal to the nucleus culminating in the transcriptional activation of genes encoding an adaptive response. Yeast Ire1p requires both protein kinase and site-specific endoribonuclease (RNase) activities to signal the UPR. In mammalian cells, two homologs, Ire1 alpha and Ire1 beta, are implicated in signaling the UPR. To elucidate the RNase requirement for mammalian Ire1 function, we have identified five amino acid residues within IRE1 alpha that are essential for RNase activity but not kinase activity. These mutants were used to demonstrate that the RNase activity is required for UPR activation by IRE1 alpha and IRE1 beta. In addition, the data support that IRE1 RNase is activated by dimerization-induced trans-autophosphorylation and requires a homodimer of catalytically functional RNase domains. Finally, the RNase activity of wild-type IRE1 alpha down-regulates hIre1 alpha mRNA expression by a novel mechanism involving cis-mediated IRE1 alpha-dependent cleavage at three specific sites within the 5' end of Ire1 alpha mRNA.
未折叠蛋白反应（UPR）是一种信号转导途径，由内质网（ER）中未折叠蛋白的积累激活。在酿酒酵母中，ER跨膜受体Ire1p将信号传递至细胞核，最终导致编码适应性反应的基因转录激活。酵母Ire1p需要蛋白激酶和位点特异性内切核糖核酸酶（RNase）活性来传递UPR信号。在哺乳动物细胞中，两个同源物Ire1α和Ire1β参与UPR信号传递。为阐明哺乳动物Ire1功能对RNase的需求，我们在IRE1α中鉴定出五个对RNase活性至关重要但对激酶活性无影响的氨基酸残基。这些突变体用于证明RNase活性是IRE1α和IRE1β激活UPR所必需的。此外，数据支持IRE1 RNase通过二聚化诱导的反式自磷酸化被激活，并且需要催化功能RNase结构域的同型二聚体。最后，野生型IRE1α的RNase活性通过一种新机制下调hIre1α mRNA表达，该机制涉及在Ire1α mRNA 5'端三个特定位点的顺式介导的IRE1α依赖性切割。
