Dysfunction or capacity shortage of the endoplasmic reticulum (ER) is cumulatively called ER stress and provokes the unfolded protein response (UPR). In various yeast species, the ER-located transmembrane protein Ire1 is activated upon ER stress and performs the splicing reaction of HAC1 mRNA, the mature form of which is translated into a transcription factor protein that is responsible for the transcriptome change on the UPR. Here we carefully assessed the splicing of HAC1 mRNA in Pichia pastoris (Komagataella phaffii) cells. We found that, inconsistent with previous reports by others, the HAC1 mRNA was substantially, but partially, spliced even without ER-stressing stimuli. Unlike Saccharomyces cerevisiae, growth of P. pastoris was significantly retarded by the IRE1-gene knockout mutation. Moreover, P. pastoris cells seemed to push more abundant proteins into the secretory pathway than S. cerevisiae cells. We also suggest that P. pastoris Ire1 has the ability to control its activity stringently in an ER stress-dependent manner. We thus propose that P. pastoris cells are highly ER-stressed possibly because of the high load of endogenous proteins into the ER.

中文翻译：

---

在没有外部压力刺激的情况下，毕赤酵母中未折叠的蛋白质反应。

内质网 (ER) 的功能障碍或容量不足被累积称为 ER 应激并引发未折叠蛋白反应 (UPR)。在各种酵母物种中，位于 ER 的跨膜蛋白 Ire1 在 ER 应激时被激活并执行HAC1 mRNA的剪接反应，其成熟形式被翻译成转录因子蛋白，负责 UPR 上的转录组变化。在这里，我们仔细评估了毕赤酵母( Komagataella phaffii ) 细胞中HAC1 mRNA的剪接。我们发现，与其他人之前的报告不一致，即使没有 ER 应激刺激，HAC1 mRNA 也基本上但部分地剪接。不像在酿酒酵母中，IRE1基因敲除突变显着阻碍了巴斯德毕赤酵母的生长。此外，与酿酒酵母细胞相比，巴斯德毕赤酵母细胞似乎将更多丰富的蛋白质推入分泌途径。我们还建议P. pastoris Ire1 有能力以依赖于 ER 压力的方式严格控制其活动。因此，我们提出P. pastoris细胞受到高度内质网应激，可能是因为内源性蛋白质高负荷进入内质网。