Young‐Ho Lee, Tomohide Saio, Mai Watabe, Motonori Matsusaki, Shingo Kanemura, Yuxi Lin, Taro Mannen, Tsubura Kuramochi, Yuka Kamada, Katsuya Iuchi, Michiko Tajiri, Kotono Suzuki, Yan Li, Yunseok Heo, Kotone Ishii, Kenta Arai, K. Bán, Mayuko Hashimoto, Shuichiro Oshita, Satoshi Ninagawa, Yoshikazu Hattori, Hiroyuki Kumeta, Airu Takeuchi, Shinji Kajimoto, Hiroya Abe, Eiichiro Mori, Takahiro Muraoka, Takakazu Nakabayashi, Satoko Akashi, Tsukasa Okiyoneda, Michele Vendruscolo, Kenji Inaba, Masaki Okumura
The endoplasmic reticulum (ER) plays crucial roles in maintaining protein quality control and regulating dynamic Ca2+ storage in eukaryotic cells. However, the proteostasis system involved in ER-mediated protein quality control has not been fully characterized. Here we show that Ca2+ triggers the condensation of PDIA6, an ER-resident disulfide isomerase and molecular chaperone, into quality control granules. In contrast to the condensation mechanism observed for proteins containing low-complexity domains, our results indicate that transient but specific electrostatic interactions occur between the first and the third folded thioredoxin-like domains of PDIA6. We further show that the PDIA6 condensates recruit proinsulin, thereby accelerating the oxidative proinsulin folding and suppressing the proinsulin aggregation inside quality control granules, essential for secretion of insulin. Lee et al. show that Ca²⁺ triggers condensates enriched with PDIA6, an ER-resident disulfide isomerase and chaperone, along with other protein disulfide isomerase family proteins and some chaperones that in turn enhance folding of proinsulin.