Engineered variants provide new insight into the structural properties important for activity of the highly dynamic, trimeric protein disulfide isomerase, PmScsC

Suppressor of copper sensitivity protein C from Proteus mirabilis (PmScsC) is a homotrimeric disulfide isomerase that plays a role in copper tolerance – a key virulence trait of the uropathogen. Each protomer of the enzyme has an N-terminal trimerisation stem (59 residues) containing a flexible linker (11 residues) connected to a thioredoxin-fold-containing catalytic domain (163 residues). Here, we characterise two PmScsC variants, PmScsCΔN and PmScsCΔLinker. PmScsCΔN, is an N-terminally truncated form of the protomer with two helices of the trimerisation stem removed, generating a protein with dithiol oxidase rather than disulfide isomerase activity. The crystal structure of PmScsCΔN reported here reveals – as expected – a monomer that is structurally similar to the catalytic domain of native PmScsC. The second variant PmScsCΔLinker was designed to remove the 11 amino acid linker and we show that it generates a protein that has neither disulfide isomerase nor dithiol oxidase activity. The crystal structure of PmScsCΔLinker reveals a trimeric arrangement, with the catalytic domains packed together very closely. Small angle X-ray scattering analysis found that native PmScsC is predominantly trimeric in solution even at low concentration, whereas PmScsCΔLinker exists as an equilibrium between monomeric, dimeric and trimeric states, with the monomeric form dominating at low concentrations. These findings increase our understanding of disulfide isomerase activity, showing how (i) oligomerisation, (ii) spacing between, and (iii) dynamic motion of, catalytic domains in PmScsC all contribute to its native function.