Endoplasmic stress sensor Ire1 is involved in cytosolic/nuclear protein quality control in Pichia pastoris cells independent of HAC1

Abstract

In eukaryotic species, dysfunction of the endoplasmic reticulum (ER), namely, ER stress, provokes a cytoprotective transcription program called the unfolded protein response (UPR). The UPR is triggered by transmembrane ER-stress sensors, including Ire1, which acts as an endoribonuclease to splice and mature the mRNA encoding the transcription factor Hac1 in many fungal species. Through analyses of the methylotrophic yeast Pichia pastoris (syn. Komagataella phaffii), we revealed a previously unknown function of Ire1. In P. pastoris cells, the IRE1 knockout mutation (ire1Δ) and HAC1 knockout mutation (hac1Δ) caused only partially overlapping gene expression changes. Protein aggregation and the heat shock response (HSR) were induced in ire1Δ cells but not in hac1Δ cells even under non-stress conditions. Moreover, Ire1 was further activated upon high-temperature culturing and conferred heat stress resistance to P. pastoris cells. Our findings cumulatively demonstrate an intriguing case in which the UPR machinery controls cytosolic protein folding status and the HSR, which is known to be activated upon the accumulation of unfolded proteins in the cytosol and/or nuclei.

Fundamental and Applicative Aspects of the Unfolded Protein Response in Yeasts

“…In agreement with the notion that P. pastoris has a robust and well-developed protein secretory system, the UPR is modestly, but clearly induced in non-stressed wild-type P. pastoris cells [89,101]. Consistently, the deletion of IRE1 or HAC1 retards the growth of P. pastoris cells even under non-stress conditions [78,89].…”

“…The target genes of the Ire1-HAC1 UPR signaling pathway are not the same among species. In P. pastoris cells, whereas the UPR highly induces genes encoding ER-located molecular chaperones and protein-folding enzymes, those encoding membrane lipid biogenesis do not appear to be prominent UPR targets [78,100]. Therefore, unlike in S. cerevisiae cells, the expansion of the ER membrane may not be an outcome of the UPR in P. pastoris.…”

“…Accordingly, IRE1and HAC1-gene knockout mutations result in the same phenotypes and do not exhibit additive or synergistic effects [76]. Whereas Tam et al [77] reported that Ire1 cleaves a few types of mRNA other than HAC1u mRNA in S. cerevisiae cells, this observation has not been reproduced by others [78].…”

“…Nevertheless, studies on some yeast or fungal species have suggested that Ire1 also plays a role(s) other than the splicing of HAC1 or HXL1 mRNA [78,103,105,110]. Fauzee et al [78] demonstrated that in P. pastoris, the IRE1-knockout and HAC1-knockout mutations result in only partially overlapping gene-expression alterations. P. pastoris Ire1 is likely to control the protein folding status in the cytosol independently of HAC1 [78].…”

“…Fauzee et al [78] demonstrated that in P. pastoris, the IRE1-knockout and HAC1-knockout mutations result in only partially overlapping gene-expression alterations. P. pastoris Ire1 is likely to control the protein folding status in the cytosol independently of HAC1 [78].…”