Ice2 promotes ER membrane biogenesis in yeast by inhibiting the conserved lipin phosphatase complex

Cells dynamically adapt organelle size to current physiological demand. Organelle growth requires membrane biogenesis and therefore needs to be coordinated with lipid metabolism. The endoplasmic reticulum (ER) can undergo massive expansion, but the underlying regulatory mechanisms are largely unclear. Here, we describe a genetic screen for factors involved in ER membrane expansion in budding yeast and identify the ER transmembrane protein Ice2 as a strong hit. We show that Ice2 promotes ER membrane biogenesis by opposing the phosphatidic acid phosphatase Pah1, called lipin in metazoa. Specifically, Ice2 inhibits the conserved Nem1‐Spo7 complex and thus suppresses the dephosphorylation and activation of Pah1. Furthermore, Ice2 cooperates with the transcriptional regulation of lipid synthesis genes and helps to maintain cell homeostasis during ER stress. These findings establish the control of the lipin phosphatase complex as an important mechanism for regulating ER membrane biogenesis.

ESCRT machinery mediates selective microautophagy of endoplasmic reticulum in yeast

ER‐phagy, the selective autophagy of endoplasmic reticulum (ER), safeguards organelle homeostasis by eliminating misfolded proteins and regulating ER size. ER‐phagy can occur by macroautophagic and microautophagic mechanisms. While dedicated machinery for macro‐ER‐phagy has been discovered, the molecules and mechanisms mediating micro‐ER‐phagy remain unknown. Here, we first show that micro‐ER‐phagy in yeast involves the conversion of stacked cisternal ER into multilamellar ER whorls during microautophagic uptake into lysosomes. Second, we identify the conserved Nem1‐Spo7 phosphatase complex and the ESCRT machinery as key components for micro‐ER‐phagy. Third, we demonstrate that macro‐ and micro‐ER‐phagy are parallel pathways with distinct molecular requirements. Finally, we provide evidence that the ESCRT machinery directly functions in scission of the lysosomal membrane to complete the microautophagic uptake of ER. These findings establish a framework for a mechanistic understanding of micro‐ER‐phagy and, thus, a comprehensive appreciation of the role of autophagy in ER homeostasis.