ALS-linked Protein Disulfide Isomerase Variants cause motor dysfunction (2016)
Figure 3

Figure 3. Effects of ALS-linked mutations in PDIA1 on the structure and enzymatic activity.

A PDI mutants form abnormal disulfide-dependent protein complexes. NSC34 cells were transiently transfected with expression vectors for V5-tagged wild-type and mutant PDIA1. After 48 h, differential disulfide-dependent interactions/aggregations of overexpressed PDI variants was assessed under reducing (+DTT) and non-reducing (-DTT) conditions in an 8% SDS-PAGE. Anti-V5 was used for detection in Western blot.

B Analysis of the PDIA1 structure to model the effects of the R300H mutation. The close association between Arg300 located to the b’ domain of PDIA1 with Trp396 located to the a’ domain adjacent to the active site motif CGHC (designated as AS in yellow) is shown in comparison to the mutated version of PDIA1R300H highlighting the same residues. A potential stabilization of the interaction between the b’ and a’ domain is shown that may be caused by the interaction of the imidazole rings of mutated His300 with Trp396.

C ALS-linked PDIA1 variants were generated as recombinant proteins and then analyzed by circular dichroism (CD). Averages for CD spectroscopic scans of PDIA1WT and mutants are shown.

D Averages for CD spectroscopic thermal denaturation of recombinant PDIA1WT and mutants.

E Representative electrophoresis analysis of Proteinase K-treated recombinant PDIA1 variants. Mass spectrometric analysis of Proteinase K digested samples from total sample and from in gel trypsin digested protein samples (arrows indicate band 1 and band 2) indicated differences in the removal of the x-linker region. Bottom panel: Ratios of band 1 to band 2 were quantified in 4 independent experiments.

F Representative fluorescence spectra of recombinant PDIA1b’xWT fragments and the equivalent ALS-linked mutants (n=6). Ratios from the two peak areas and change compared to PDIA1 b’xWT. Both mutants analyzed show a significant shift in peak position compared to wild-type, but in opposite directions, suggesting that PDIA1D292N shifts equilibrium towards the capped version (x region over the binding pocket) and the PDIA1R300H mutant towards the uncapped version (Nguyen et al, 2008).

G The activity of recombinant PDIA1 was measured in vitro using a BPTI refolding assay following by mass spectrometry analysis. The percentages of different BPTI species was calculated (6H, fully reduced; 1S, one disulfide bond; 2S, two disulfide bonds; 3S, three disulfide bonds) at time point 2.5 min in four independent experiments.

H Measurement of H2O2 levels at the ER lumen of living cells. Left panel: Reduced PDIs can be oxidized by oxidized ERO1Lα that then transfers electrons to molecular oxygen (O2), generating hydrogen peroxide (H2O2) as a product of PDIs activity. Right panel: NSC34 cells were transiently co-transfected with ER luminal HyPer sensor and indicated PDIs. After 48 h the 490/420 nm fluorescence ratio was recorded for 2 min under basal conditions. Means and SEM derived from all cells per condition (n=55-74) monitored in four independent experiments are shown.

I HEK293T cells were transfected with expression vectors for V5-tagged wild-type and mutant PDIA1, as well as empty vector. After 48 h, V5-tagged proteins were immunoprecipitated and eluted with V5 peptide. The interaction with endogenous ERO1Lα was analyzed by western blot. The inputs and elutions are shown as control. Right panel: Quantification of the degree of interaction is presented.

Statistical analyses were performed using Student’s t-test in panels F, G and I or one-way ANOVA and Bonferroni´s post hoc tests in panel H. Mean ± SEM with p values: n.s., p > 0.05; *, p ≤ 0.05; **, p ≤ 0.01; ***, p ≤ 0.001.