Preparation of Oxidized and Reduced PTP4A1 for Structural and Functional Studies

The formation of a reversible disulfide bond between the catalytic cysteine and a spatially neighboring cysteine (backdoor) in protein tyrosine phosphatases (PTPs) serves as a critical regulatory mechanism for maintaining the activity of protein tyrosine phosphatases. The failure of such protection results in the formation of irreversibly oxidized cysteines into sulfonic acid in a highly oxidative cellular environment in the presence of free radicals. Hence, it is important to develop methods to interconvert PTPs into reduced and oxidized forms to understand their catalytic function in vitro. Protein tyrosine phosphatase 4A type 1 (PTP4A1), a dual-specificity phosphatase, is catalytically active in the reduced form. Unexpectedly, also its oxidized form performs a key biological function in systemic sclerosis (SSc) by forming a kinase–phosphatase complex with Src kinases. Thus, we developed simple and efficient protocols for producing oxidized and reduced PTP4A1 to elucidate their biological function, which can be extended to study other protein tyrosine phosphatases and other recombinantly produced proteins.