MS analysis of an FGE-PDI complex

Multiple Sulfatase Deficiency (MSD) is a fatal, inherited lysosomal storage disorder characterised by reduced activities of all cellular sulfatases in patients. Sulfatases require a unique post-translational modification of an active site cysteine to formylglycine that is catalysed by the Formylglycine Generating Enzyme (FGE). FGE mutations leading to MSD affect the intracellular protein stability that determines residual enzyme activity and defines disease severity in MSD patients. Here, we show that Protein Disulfide Isomerase (PDI) plays a pivotal role in the recognition and quality control of MSD-causing FGE variants. Overexpression of PDI reduces the residual activity of unstable FGE variants, whereas inhibition of PDI function rescues the residual activity of sulfatases in MSD fibroblasts. Analysing a covalent complex consisting of PDI and a FGE variant by LC-MALDI mass spectrometry allowed to determine the molecular signature for FGE recognition by PDI. Our findings highlight the role of PDI as a disease modifier in MSD, which may be relevant also for other ER-associated protein folding pathologies.