Figure 2.

Proper ER membrane expansion requires UPR signaling. (A) Wild-type, hac1Δ, and ire1Δ cells expressing Sec63-GFP (SSY139, SSY161, and SSY467) treated with DTT for 2 h. Expansion of the peripheral ER is impaired in UPR mutants, and aberrant cytoplasmic ER patches form at the cell periphery and adjacent to the nucleus (arrows). (B) Quantification of ER expansion from images was obtained as in A. Statistical significance compared with the wild type at the same time point of P ≤ 10⁻⁶ (***) is shown. Error bars indicate SEM. (C) Sec63-GFP Rtn1-Cherry hac1Δ cells (SSY429) untreated or treated with DTT for 2 h. The cytoplasmic ER patches in DTT-treated cells contain both Sec63 and Rtn1. (D) Cytoplasmic ER patches induced by ER stress in hac1Δ cells are tangles of smooth ER. Electron micrographs of hac1Δ cells (SSY161) treated with DTT for 2 h. (top) Low magnification images of hac1Δ cells with cytoplasmic ER patches (arrows), which are found either at the cell periphery or close to the nucleus, are shown. (bottom) Sequential 50-nm sections are shown at a higher magnification corresponding to the boxed area (top). The 0-nm image is the same as shown in the top panel. The ER tangle shown consists of numerous randomly arranged ribosome-free elements. N, nucleus; V, vacuole. Bars: (A and C) 2 µm; (D) 250 nm.