Yip1A is responsible for the phosphorylation of IRE1 via high-order assembly of Ire1 molecules.

HeLa cells were transfected with each siRNA for 24 hr, and then treated with Tm to induce the UPR. Cell lysates were prepared at the indicated time points and analyzed by Western blotting. (A) Representative immunoblots for pIRE1, spliced-XBP1, and β-tubulin. β-tubulin was used for normalization. The intensity of the bands was quantified using the MultiGauge software. (B-D) Relative protein levels of pIRE1 (B), relative mRNA levels of spliced-XBP1 (C), and relative protein levels of spliced-XBP1 (C) in control (solid circles) and Yip1A-knockdown (open circles) cells. The protein or mRNA levels in control cells at the beginning of the Tm treatment were assigned the value 1. Data are means ± SD from three independent experiments. (E) Representative immunoblots for Sar1, Sec23, Sec24D, and GAPDH, and relative protein levels of Sar1, Sec23, and Sec24D in control (solid circles), Yip1A-knockdown (open circles), and IRE1-knockdown (solid gray circles) cells. GAPDH was used for normalization. The intensity of the bands was quantified using the MultiGauge software, and the results are shown in the line graphs. The protein levels in control cells at the beginning of the Tm treatment were assigned the value 1. Data are means ± SD from three independent experiments. (F) Representative confocal micrographs of control (upper panels) or Yip1A-knockdown (lower panels) cells during the Tm treatment. Fixed cells at the indicated time points were stained for pIRE1. Cells are outlined with white dashed lines. Scale bars are 10 μm. The numbers of pIRE1 foci per cell were counted, and shown in the line graph. Data are means ± SD (N = 30). (G) Representative immunoblot for pIRE1 after native PAGE, showing two high-order complexes of pIRE1 (pIRE1-I and pIRE1-II). Lane ‘S’ represents lysate from HeLa cells transfected with control scramble siRNA, and lane ‘Y’ represents lysate from HeLa cells transfected with Yip1A siRNA. Numbers on the left-hand side correspond to the standard molecular weight. The intensity of the bands was quantified by using the MultiGauge software, and the results are shown in the bar graphs. Protein levels in control cells at the beginning of the Tm treatment were assigned the value 1. Data are means ± SD from three independent experiments. **: p<0.01.