title: SHMT1 and SHMT2 are functionally redundant in nuclear de novo thymidylate biosynthesis.
abstract: The three enzymes that constitute the de novo thymidylate synthesis pathway in mammals, cytoplasmic serine hydroxymethyltransferase (SHMT1), thymidylate synthase (TYMS) and dihydrofolate reductase (DHFR) undergo sumoylation and nuclear import during S-phase. In this study, we demonstrate that purified intact mouse liver nuclei convert dUMP to dTMP in the presence of NADPH and serine. Neither nuclear extracts nor intact nuclei exposed to aminomethylphosphonate, a SHMT inhibitor, exhibit thymidylate synthesis activity. Nuclei isolated from Shmt1(-/-) mouse livers retained 25% of thymidylate synthesis activity exhibited by nuclei isolated from wild type mice. This residual activity was due to the presence of a cytoplasmic/nuclear isozyme of SHMT encoded by Shmt2. Shmt2 is shown to encode two transcripts, one which encodes a protein that localizes exclusively to the mitochondria (SHMT2), and a second transcript that lacks exon 1 and encodes a protein that localizes to the cytoplasm and nucleus during S-phase (SHMT2alpha). The ability of Shmt2 to encode a cytoplasmic isozyme of SHMT may account for the viability of Shmt1(-/-) mice and provide redundancy that permitted the expansion of the human SHMT1 L474F polymorphism that impairs SHMT1 sumoylation and nuclear translocation.
authors.affiliation: Graduate Field of Biochemistry, Molecular and Cellular Biology, Cornell University, Ithaca, New York, USA.
authors.forename: Donald D
authors.initials: DD
authors.lastname: Anderson
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Stover
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D016466:CHO Cells; D002467:Cell Nucleus / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D012696:Glycine Hydroxymethyltransferase / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D008099:Liver / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D009249:NADP / Q000737:chemistry; D041681:NIH 3T3 Cells; D012694:Serine / Q000737:chemistry; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism; D013942:Thymine Nucleotides / Q000737:chemistry*
chemical_list: D013942:Thymine Nucleotides; D012694:Serine; D009249:NADP; D013762:Tetrahydrofolate Dehydrogenase; D012696:Glycine Hydroxymethyltransferase; C520541:SHMT protein, human; C024157:thymidine 5'-triphosphate