Proper folding of nascent proteins in the cell is accelerated by chaperones. They serve to stabilize protein domains and overall structure. One such family of protein-folding catalysts comprise dithiol-disulfide oxidoreductases of the protein disulfide isomerase (PDI) family. PDIs typically contain two different types of thioredoxin-like domains, catalytic (a) and non-catalytic (b). PDIA1 is a oxidoreductase and molecular chaperone that localizes in the lumen of the ER, and accounts for roughly 0.8% of total cellular protein It is an important folding catalyst for secretory pathway proteins. PDIA1 contains two active-site domains (a and a′), each containing a Cys-Gly-His-Cys (CGHC) active-site motif. KSC-34 is also a selective and potent a-site inhibitor of PDIA1 with an IC₅₀ of 3.5 μM. It displays a 30-fold selectivity for a domain over a′ domain and displays high selectivity for PDIA1 in complex proteomes with minimal engagement of other members of the PDI family.

KSC-34 displays time-dependent inhibition of PDIA1 reductase activity in vitro. The k_inact/ K_I is 9.66 × 10³ M^-1 s^-1. Moreover, KSC-34 decreases destabilized ALLC secretion through a mechanism dependent on PDIA1.  In addition, it contains a (4-phenylbutyl)methylamine diversity element for optimized binding to the active site of the a domain of PDIA1 with a chloroacetamide electrophile for covalent modification of C53 on PDIA1. Importantly, KSC-34 is unique in that it displays selective binding to the a domain of PDIA1.

In summary, KSC-34 is a useful tool compound to further interrogate the cellular functions of the a domain of PDIA1. Additionally, it is a useful tool to explore the therapeutic value of site-selective PDI inhibitors.

Reference:

Cole KS, et al. Biochemistry. 2018;57(13):2035-2043.