UBX domain-containing proteins are involved in lipid homeostasis and stress responses in Pichia pastoris.

Ubiquitin regulatory X (UBX) domain-containing proteins constitute a family of proteins and are substrate adaptors of AAA ATPase Cdc48. UBX proteins can bind to the N-terminal region of Cdc48 to perform endoplasmic reticulum associated protein degradation (ERAD). In this study, we identified two UBX domain-containing proteins, Ubx1 and Ubx2, in Pichia pastoris and found that the two proteins could recover the growth defect of Saccharomyces cerevisiae in ubx2Δ. Our results revealed that Ubx1 and Ubx2 play critical roles in synthesis of unsaturated fatty acids by affecting Spt23. In addition, the results demonstrated that both Ubx1 and Ubx2 are involved in lipid droplet formation and protein degradation. Deletion of UBX1 led to increased sensitivity to oxidative stress and disruption of UBX2 impaired cell viability under osmotic stress. The phenotypes of ubx1Δ+UBX2, ubx2Δ+UBX1 and ubx1Δubx2Δ and RNA-seq data suggested that Ubx1 and Ubx2 play different roles in cell functions, and the roles of Ubx1 may be more numerous than Ubx2. In summary, our findings provide new insights into the relationship between lipid homeostasis and cell functions in the oil-producing organism P. pastoris.