The 5-formyl-tetrahydrololate (5-formyl-THF) is an inhibitor of several folate-dependent enzymes and plays a role in folic acid-dependent one-carbon metabolism. It is synthesized in vivo by the Serine-Hydroxymethyl-Transferase (SHMT), an enzyme expressed at steady level, which activity is inhibited by 5-formyl-THF itself. The 5,10-methenyl-tetrahydrofolate-synthetase (MTHFS) is the only enzyme that metabolize the 5-formyl-THF, catalyzing the ATP-dependent conversion to 5,10-methenyl-THF. The combined activities of MTHFS and SHMT constitute a futile cycle that may buffer the 5-formyl-THF concentration. The MTHFS is a 28 kDa polypeptide. Its transcripts are detectable in all tissues, but so far no evidence of modulation of its expression has been reported. The MTHFS gene is located in 15q24.3. It is organized in three exons. The two introns are > 7Kb in length. The gene has no TATA-box, and transcription is initiated at multiple sites at 14-23 nt upstream the translation start codon. Two distinct poly(A) sites have been identified downstream the coding sequence. The expression kinetics of MTHFS enzyme and transcript has been studied in phytohaemagglutinin (PHA)-activated human lymphocytes. Quiescent cells have low transcription while the enzyme level is relatively high. Upon PHA stimulation the 28 kDa protein level drops down, whereas a 20 kDa isoform is expressed. This smaller isoform is due to the use of an alternative transcription initiation site located 12 nt downstream the major translation start codon, within an initiator element consensus sequence. The role and function of this 20 kDa protein is not known. As the activated lymphocytes enter the cell cycle, the 20 kDa isoform is replaced by the 28 kDa MTHFS enzyme. This evidence indicates that the MTHFS expression is modulated during the proliferation process, and this may in turn determine changes in the intracellular 5-formyl-THF level. Regulating the intracellular level of 5-formyl-THF, the MTHFS activity may influence the methyl-THF concentration and the homocysteine remethylation.
Genomic organization and expression of the MTHFS gene
RUBINI, Michele;BONONI, Ilaria;CALZOLARI, Elisa
2000
Abstract
The 5-formyl-tetrahydrololate (5-formyl-THF) is an inhibitor of several folate-dependent enzymes and plays a role in folic acid-dependent one-carbon metabolism. It is synthesized in vivo by the Serine-Hydroxymethyl-Transferase (SHMT), an enzyme expressed at steady level, which activity is inhibited by 5-formyl-THF itself. The 5,10-methenyl-tetrahydrofolate-synthetase (MTHFS) is the only enzyme that metabolize the 5-formyl-THF, catalyzing the ATP-dependent conversion to 5,10-methenyl-THF. The combined activities of MTHFS and SHMT constitute a futile cycle that may buffer the 5-formyl-THF concentration. The MTHFS is a 28 kDa polypeptide. Its transcripts are detectable in all tissues, but so far no evidence of modulation of its expression has been reported. The MTHFS gene is located in 15q24.3. It is organized in three exons. The two introns are > 7Kb in length. The gene has no TATA-box, and transcription is initiated at multiple sites at 14-23 nt upstream the translation start codon. Two distinct poly(A) sites have been identified downstream the coding sequence. The expression kinetics of MTHFS enzyme and transcript has been studied in phytohaemagglutinin (PHA)-activated human lymphocytes. Quiescent cells have low transcription while the enzyme level is relatively high. Upon PHA stimulation the 28 kDa protein level drops down, whereas a 20 kDa isoform is expressed. This smaller isoform is due to the use of an alternative transcription initiation site located 12 nt downstream the major translation start codon, within an initiator element consensus sequence. The role and function of this 20 kDa protein is not known. As the activated lymphocytes enter the cell cycle, the 20 kDa isoform is replaced by the 28 kDa MTHFS enzyme. This evidence indicates that the MTHFS expression is modulated during the proliferation process, and this may in turn determine changes in the intracellular 5-formyl-THF level. Regulating the intracellular level of 5-formyl-THF, the MTHFS activity may influence the methyl-THF concentration and the homocysteine remethylation.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
