PMID:16769880

Citation	Yamada, K, Gravel, RA, Toraya, T and Matthews, RG (2006) Human methionine synthase reductase is a molecular chaperone for human methionine synthase. Proc. Natl. Acad. Sci. U.S.A. 103:9476-81
Abstract	Sustained activity of mammalian methionine synthase (MS) requires MS reductase (MSR), but there have been few studies of the interactions between these two proteins. In this study, recombinant human MS (hMS) and MSR (hMSR) were expressed in baculovirus-infected insect cells and purified to homogeneity. hMSR maintained hMS activity at a 1:1 stoichiometric ratio with a K(act) value of 71 nM. Escherichia coli MS, however, was not activated by hMSR. Moreover, hMS was not significantly active in the presence of E. coli flavodoxin and flavodoxin reductase, which maintain the activity of E. coli MS. These results indicate that recognition of MS by their reductive partners is very strict, despite the high homology between MS from different species. The effects of hMSR on the formation of hMS holoenzyme also were examined by using crude extracts of baculovirus-infected insect cells containing hMS apoenzyme (apoMS). In the presence of MSR and NADPH, holoenzyme formation from apoMS and methylcobalamin was significantly enhanced. The observed stimulation is shown to be due to stabilization of human apoMS in the presence of MSR. Apoenzyme alone is quite unstable at 37 degrees C. MSR also is able to reduce aquacobalamin to cob(II)alamin in the presence of NADPH, and this reduction leads to stimulation of the conversion of apoMS and aquacobalamin to MS holoenzyme. Based on these findings, we propose that MSR serves as a special chaperone for hMS and as an aquacobalamin reductase, rather than acting solely in the reductive activation of MS.
Links	PubMed PMC1480432 Online version:10.1073/pnas.0603694103
Keywords	5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase/genetics; 5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase/isolation & purification; 5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase/metabolism; Apoenzymes/metabolism; Ferredoxin-NADP Reductase/genetics; Ferredoxin-NADP Reductase/isolation & purification; Ferredoxin-NADP Reductase/metabolism; Holoenzymes/metabolism; Humans; Methionine/biosynthesis; Molecular Chaperones/genetics; Molecular Chaperones/isolation & purification; Molecular Chaperones/metabolism; Oxidation-Reduction; Spectrum Analysis; Vitamin B 12/analogs & derivatives; Vitamin B 12/pharmacology
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Annotations

Gene product	Qualifier	GO Term	Evidence Code	with/from	Aspect	Extension	Notes	Status
HUMAN:MTRR	enables	GO:0050444: aquacobalamin reductase (NADPH) activity	ECO:0000314: direct assay evidence used in manual assertion		F		Seeded From UniProt	complete
HUMAN:MTRR		GO:0050444: aquacobalamin reductase (NADPH) activity	ECO:0000314:		F		Figure 5: used differential molecular extinction coefficient of aqCbl to cob(II)alamin at 525 nm in the presence of hMSR	complete CACAO 6879
HUMAN:METH		GO:0008705: methionine synthase activity	ECO:0000314:		F		Figure 2: shows methionine formed by hMS in the presence of increasing concentrations of hMSR	complete CACAO 6995
HUMAN:METH	enables	GO:0008705: methionine synthase activity	ECO:0000314: direct assay evidence used in manual assertion		F		Seeded From UniProt	complete

References

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