title: S-adenosylmethionine decarboxylase degradation by the 26 S proteasome is accelerated by substrate-mediated transamination.
abstract: The short-lived enzyme S-adenosylmethionine decarboxylase uses a covalently bound pyruvoyl cofactor to catalyze the formation of decarboxylated S-adenosylmethionine, which then donates an aminopropyl group for polyamine biosynthesis. Here we demonstrate that S-adenosylmethionine decarboxylase is ubiquitinated and degraded by the 26 S proteasome in vivo, a process that is accelerated by inactivation of S-adenosylmethionine decarboxylase by substrate-mediated transamination of its pyruvoyl cofactor. Proteasome inhibition in COS-7 cells prevents the degradation of S-adenosylmethionine decarboxylase antigen; however, even brief inhibition of the 26 S proteasome caused substantial losses of S-adenosylmethionine decarboxylase activity despite accumulation of S-adenosylmethionine decarboxylase antigen. Levels of the enzyme's substrate (S-adenosylmethionine) increased rapidly after 26 S proteasome inhibition, and this increase in substrate level is consistent with the observed loss of activity arising from an increased rate of inactivation by substrate-mediated transamination. Evidence is also presented that this substrate-mediated transamination accelerates normal degradation of S-adenosylmethionine decarboxylase, as the rate of degradation of the enzyme was increased in the presence of AbeAdo (5'-([(Z)-4-amino-2-butenyl]methylamino]-5'-deoxyadenosine) (a substrate analogue that transaminates the enzyme); conversely, when the intracellular substrate level was reduced by methionine deprivation, the rate of degradation of the enzyme was decreased. Ubiquitination of S-adenosylmethionine decarboxylase is demonstrated by isolation of His-tagged AdoMetDC (S-adenosylmethionine decarboxylase) from COS-7 cells co-transfected with hemagglutinin-tagged ubiquitin and showing bands that were immunoreactive to both anti-AdoMetDC antibody and anti-hemagglutinin antibody. This is the first study to demonstrate that AdoMetDC is ubiquitinated and degraded by the 26 S proteasome, and substrate-mediated acceleration of degradation is a unique finding.
authors.affiliation: Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, 500 University Drive, Hershey, PA 17033, USA.
authors.forename: Azmi
authors.initials: A
authors.lastname: Yerlikaya
authors.forename: Bruce A
authors.initials: BA
authors.lastname: Stanley
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry; D012437:Adenosylmethionine Decarboxylase / Q000378:metabolism*; D000818:Animals; D015153:Blotting, Western; D019556:COS Cells; D002384:Catalysis; D003513:Cycloheximide / Q000494:pharmacology; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D006801:Humans; D007930:Leucine / Q000737:chemistry; D008280:Magnetics; D008715:Methionine / Q000737:chemistry; D009097:Multienzyme Complexes / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000378:metabolism*; D010957:Plasmids / Q000378:metabolism; D011233:Precipitin Tests; D046988:Proteasome Endopeptidase Complex; D012685:Sepharose / Q000737:chemistry; D013997:Time Factors; D000637:Transaminases / Q000378:metabolism*; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism
chemical_list: D009097:Multienzyme Complexes; D025801:Ubiquitin; D000255:Adenosine Triphosphate; D012685:Sepharose; D003513:Cycloheximide; D008715:Methionine; D000637:Transaminases; D010447:Peptide Hydrolases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease; D012437:Adenosylmethionine Decarboxylase; D007930:Leucine