PDIb' family, ERp44 subfamily, second redox inactive TRX-like domain b'; ERp44 is an endoplasmic reticulum (ER)-resident protein, induced during stress, involved in thiol-mediated ER retention. It contains an N-terminal TRX domain with a CXFS motif followed by two redox inactive TRX-like domains, homologous to the b and b' domains of PDI. Through the formation of reversible mixed disulfides, ERp44 mediates the ER localization of Ero1alpha, a protein that oxidizes protein disulfide isomerases into their active form. ERp44 also prevents the secretion of unassembled cargo protein with unpaired cysteines. ERp44 also modulates the activity of inositol 1,4,5-triphosphate type I receptor (IP3R1), an intracellular channel protein that mediates calcium release from the ER to the cytosol. Similar to PDI, the b' domain of ERp44 is likely involved in substrate recognition and may be the primary binding site.
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