MTHFD1 and Nuclear Thymidylate Biosynthesis

Abstract

Thymidylate (dTMP) is the only DNA nucleotide formed at the sites of DNA replication. dTMP is synthesized by a multi‐enzyme complex, which includes serine hydroxymethylase 1 (SHMT1) and methylenetetrahydrofolate dehydrogenase 1 (MTHFD1). When dTMP synthesis is impaired, dU is incorporated into DNA, which can lead to DNA strand breaks. SHMT1 has been shown to act as a scaffold to anchor the thymidylate biosynthesis complex to the nuclear lamina and MTHFD1 supplies folate‐activated single carbons for dTMP synthesis. Both proteins have been shown to be SUMOylated in vitro and in vivo. Protein SUMOylation serves many functions including trafficking proteins to the nucleus, protecting proteins from ubiquitin dependent proteasome degradation, and assembling proteins into complexes. It is hypothesized that the non‐covalent interaction between SUMOylated proteins and SUMO interacting motifs (SIMs) on other proteins facilitate multiprotein complex formation and stabilization. In this study, MTHFD1 is shown to localize to the nucleus during S‐phase in HeLa cells and when MTHFD1 is mutated at its SUMOylation site, it's nuclear localization is impaired. Furthermore, we have identified a SIM on SHMT1 that may function in complex formation and may be regulated by CK2 phosphorylation. We also demonstrate that arsenic exposure affects SHMT1 and MTHFD1 SUMOylation, causes genome instability, and that the effects of folate‐depletion and arsenic on genome stability are additive. Collectively, these studies reveal the factors that regulate the metabolic complex responsible for dTMP biosynthesis to maintain genome stability.