Ubiquitin-like protein MNSFβ covalently binds to cytosolic 10-formyltetrahydrofolate dehydrogenase and regulates thymocyte function

MNSF beta is a ubiquitously expressed member of the ubiquitin-like family that has been involved in various biological functions. Previous studies have demonstrated that MNSF beta covalently binds to various target proteins including Bcl-G, a proapoptotic protein. In this study, we purified a 115 kDa MNSF beta adduct from murine liver lysates by sequential chromatography on DEAE and anti-MNSF beta IgG-conjugated Sepharose in the presence of ATP. MALDI-TOF MS fingerprinting revealed that this MNSF beta adduct consists of an 8.5 kDa MNSF beta and 10-formyltetrahydrofolate dehydrogenase (FDH), an abundant enzyme of folate metabolism. Interestingly, MNSF beta preferably binds to cytosolic but not mitochondrial FDH. Fingerprinting analysis of the MNSF beta adduct demonstrate that MNSF beta conjugates to cytosolic FDH with a linkage between the C-terminal Gly74 and Lys72. The 115 kDa MNSF beta/FDH complex was not expressed in any of the tissues examined, indicating that this adduct formation is not ubiquitous. We found that MNSF beta/FDH complex formation was induced by dexamethasone in thymocytes. Double knockdown of MNSF beta and FDH strongly reduced dexamethasone-induced apoptosis. Collectively, MNSF beta/FDH complex formation may positively regulate apoptosis in thymocytes. (C) 2015 Elsevier Inc. All rights reserved.