Chaperones as mediators of ER stress induced protein reflux

Abstract

Protein Disulfide Isomerase-A1 (PDIA1) is a dithiol-disulfide oxidoreductase chaperone with a major canonical function of redox protein folding (disulfide bond formation/isomerization) in nascent proteins at the endoplasmic reticulum (ER). Thus,PDIA1 exerts a central role in ER-associated proteostasis and redox balance. The presence of PDIA1 in different subcellular locations has been documented, including mitochondria, nucleus and cytosol and also at the cell surface and extracellular milieu (pecPDIA1). At these non-ER locations PDIA1 function seems to differ from that accounted to proteostasis. Our group has consolidated a new concept, namely the existence of a cytoplasmic PDIA1 pool associated with specific effects on redox-regulated cytoskeletal proteins and mechanosignaling. However, the possible cytoplasmic location of PDIA1, as well as other thiol isomerases, remains controversial. Despite several indirect lines of evidences, the characterization of the cytosolic pool and its physiological relevance remain to be elucidated. These are important questions that are current under investigation in our group (post-doctoral work) in which this proposal is associated to. Preliminary results indicate that there is a basal pool of PDIA1 in the cytosol which increases during ER stress. These data confirm and expand recent findings in the literature that uncovered in yeast model an ER stress induced "protein reflux" system that delivers intact, folded ER luminal proteins back to the cytosol. It requires distinct ER-resident and cytosolic chaperones and cochaperones. Therefore, general question at stake here is whether chaperones homologous to those ones described for yeast are involved in the cytosolic PDIA1 traffic in mammalian cells. Our specific aims are: 1) To investigate, through loss of function experiments, whether the transport of PDIA1 from the ER lumen to the cytosol requires the function of specific chaperones; 2) To investigate whether ER stress induced cells in which ER-cytosol traffic is eventually reduced by the chaperone loss of function exhibit impairment in viability/proliferation. These data may also generate relevant advances for understanding the cytosolic redox network, as well new investigative tools and potential therapeutic targets related to novel interactions.