Identification of the PDI-Family Member ERp90 as an Interaction Partner of ERFAD

In the endoplasmic reticulum (ER), members of the protein disulfide isomerase (PDI) family perform critical functions during protein maturation. Herein, we identify the previously uncharacterized PDI-family member ERp90. In cultured human cells, we find ERp90 to be a soluble ER-luminal glycoprotein that comprises five potential thioredoxin (Trx)-like domains. Mature ERp90 contains 10 cysteine residues, of which at least some form intramolecular disulfides. While none of the Trx domains contain a canonical Cys-Xaa-Xaa-Cys active-site motif, other conserved cysteines could endow the protein with redox activity. Importantly, we show that ERp90 co-immunoprecipitates with ERFAD, a flavoprotein involved in ER-associated degradation (ERAD), through what is most likely a direct interaction. We propose that the function of ERp90 is related to substrate recruitment or delivery to the ERAD retrotranslocation machinery by ERFAD.

在内质网（endoplasmic reticulum, ER）中，蛋白质二硫键异构酶（protein disulfide isomerase, PDI）家族成员在蛋白质成熟过程中发挥关键功能。本文中，我们鉴定出此前未被表征的PDI家族成员ERp90。在培养的人类细胞中，我们发现ERp90是一种可溶性的ER腔糖蛋白，包含五个潜在的硫氧还蛋白（thioredoxin, Trx）样结构域。成熟的ERp90含有10个半胱氨酸残基，其中至少部分形成分子内二硫键。尽管所有Trx样结构域均不包含典型的Cys-Xaa-Xaa-Cys活性位点基序，但其他保守的半胱氨酸或可赋予该蛋白氧化还原活性。重要的是，我们证实ERp90可与ERFAD——一种参与内质网相关降解（ER-associated degradation, ERAD）的黄素蛋白——共免疫沉淀，且二者间的相互作用极有可能为直接相互作用。我们推测ERp90的功能与ERFAD对ERAD逆向转运机器的底物招募或递送相关。