Reexamining the essentiality of Pdi1 in yeast – A PDI1 knockout is viable in Komagataella phaffii and still produces recombinant disulfide bonded proteins

Protein disulfide isomerase 1 (Pdi1) and Ero1 form the main oxidative folding axis in the endoplasmic reticulum (ER). Despite having additional PDI family members, Pdi1 has long ago been determined to be essential in baker’s yeast and this concept was inferred to other yeast species. In this study, we reexamine the gene essentiality of Pdi1 in the methylotrophic yeast Komagataella phaffii . Strikingly, the absence of Pdi1 does not cause lethality, but even allows for folding and secretion of heterologous proteins in a homogeneous redox state. Remarkably, while sensitivity to externally added folding stressors, such as tunicamycin and DTT is increased in pdi1 Δ knock-out cells, in non-stressed growth conditions they do not display upregulation of folding stress markers, such as the master chaperone Kar2 or the unfolded protein response regulator Hac1, suggesting homeostatic adaptation.