SHMT1 and SHMT2 are functionally redundant in nuclear de novo thymidylate biosynthesis

• Published in: PloS one 2009-06, Vol.4 (6), p.e5839-e5839
• Main Authors: Anderson, Donald D, Stover, Patrick J
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amino acids; Animals; Base Sequence; Binding sites; Biochemistry; Biochemistry/ and Repair; Biosynthesis; Caenorhabditis elegans; Carbon; Cell Biology/Nuclear Structure and Function; Cell Nucleus - metabolism; Cellular biology; CHO Cells; Chromatography; Coding; Cricetinae; Cricetulus; Cytoplasm; Dehydrogenases; Developmental Biology/Developmental Molecular Mechanisms; Dihydrofolate reductase; DNA methylation; Enzymes; Glycine Hydroxymethyltransferase - metabolism; HeLa Cells; Homocysteine; Humans; Liver; Liver - metabolism; Mass spectrometry; Metabolism; Mice; Mitochondria; Molecular Biology/DNA; Molecular Sequence Data; NADP; NADP - chemistry; Neuroblastoma; NIH 3T3 Cells; Nuclear transport; Nuclei; Physiological aspects; Polymorphism; Redundancy; Regulation; Scientific imaging; Serine; Serine - chemistry; SUMO protein; Tetrahydrofolate dehydrogenase; Tetrahydrofolate Dehydrogenase - metabolism; Thymidylate synthase; Thymine Nucleotides - chemistry; Transcription; Translocation; Viability; Vitamin B
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0005839

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.