In reduced PDI, a, b, and b’ domains are arranged on the same plane, and only the a’ domain twists outward by ~45° ( Figure 1A, right). Crystal structures of reduced and oxidized PDI from human revealed redox-dependent conformational switching. Redox-dependent client binding/release by PDI has been reported for several substrates, including cholera toxin, antigen peptides, and the α-subunit of prolyl-4-hydroxylase (P4-H). In addition to conformational changes caused by substrate binding to the b’ domain, the redox state of the active site, especially in the PDI a’ domain, undergoes striking conformational changes that affect substrate selectivity. This conformational change leads to closure of the substrate-binding pocket in the b’ domain, preventing PDI from binding to other clients. Dynamic light scattering and small-angle X-ray scattering (SAXS) analyses indicate that binding of the inhibitor bisphenol-A to the substrate-binding sites of PDI b’ induces significant rearrangement of its N-terminal Trx-like domain, resulting in a more compact overall structure of PDI.
To recruit clients, the human PDI b’ domain provides the principal substrate-binding sites based primarily on a hydrophobic pocket including Phe240, Phe249, and Phe304. Both a and a’ domains contain a Cys-X-X-Cys motif as a redox-active site essential for thiol-disulfide exchange reactions with substrate proteins. PDI consists of four thioredoxin (Trx)-like domains named a, b, b’, and a’, which form a compact U-shape ( Figure 1A).
PDI (also known as PDIA1 and P4HB) is identified as an enzyme that promotes oxidative folding and acts as a molecular chaperone. Redox-Dependent Conformational Changes of PDI This review summarizes recent findings and the underlying mechanism by which PDI family members catalyze client folding and assembly to maintain proteostasis in the ER.Ģ.1. These PDI family proteins target a wide range of substrates such as antibodies and major histocompatibility complex (MHC) class-I. The protein disulfide isomerase (PDI) family, of which there are more than 20 types present in the ER, forms a sophisticated proteostasis network in this organelle. The oxidized and reduced glutathione ratio in the ER ranges from 1:1 to 1:3, and this redox environment helps to promote oxidative folding reactions. The newly synthesized proteins cotranslationally inserted into the ER are structurally immature, and oxidative folding is completed while they are protected against aggregation in the crowded ER environment, after which they are transported to the Golgi apparatus. Disulfide bond formation can drive folding, inhibit unfolding, and stabilize oligomeric protein assemblies. Meanwhile, approximately one-third of all newly synthesized proteins such as antibodies and insulin are inserted into the endoplasmic reticulum (ER). Over several decades, many researchers have gained substantial insight into protein homeostasis (proteostasis) networks in the cytosol. Therefore, cell viability is maintained by chaperone networks that control protein folding, assembly, antiaggregation, disaggregation, and degradation. TRiC also prevents aggregation and toxicity of proteins linked to amyloid neurodegenerative disorders. The eukaryotic chaperonin, tailless complex polypeptide 1 ring complex (TRiC)/chaperonin-containing tailless complex polypeptide 1 (CCT), an ATP-dependent chaperone in the cytosol, is required to ensure native folding and subunit assembly of tubulin. Tubulin, a heterodimer composed of α and β subunits, has been employed as a model for protein folding and assembly research. Although numerous studies on single-domain protein folding have been reported, there are relatively few studies on how interactions between subunits/domains affect protein folding and assembly to obtain native quaternary structures. These proteins must adopt specific three-dimensional structures to perform their biological functions. A typical mammalian cell expresses approximately 20,000 different proteins, more than 70% of which contain multiple domains.
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