a b s t r a c tPutrescine N-methyltransferase (PMT) catalyses S-adenosylmethionine (SAM)-dependent methylation of putrescine in tropane alkaloid biosynthesis. PMT presumably evolved from the ubiquitous spermidine synthase (SPDS). SPDS protein structure suggested that only few amino acid exchanges in the active site were necessary to achieve PMT activity. Protein modelling, mutagenesis, and chimeric protein construction were applied to trace back evolution of PMT activity from SPDS. Ten amino acid exchanges in Datura stramonium SPDS dismissed the hypothesis of facile generation of PMT activity in existing SPDS proteins. Chimeric PMT and SPDS enzymes were active and indicated the necessity for a different putrescine binding site when PMT developed.
SummaryCruciferin, a main storage protein of rapeseeds has been investigated by means of chromatofocusing, isoelectric focusing and electrophoresis in polyacrylamide gels. It was shown, that the half molecules of this oligomeric protein differ from each other by subunit composition. There are specific forms of cruciferin. At low ionic strength the half molecules, or subunits and/or polypeptide chains under denaturing conditions, aggregate to relatively stable complexes, evidently, by means of hydrophobic interactions.
ZusammenfassungZum Verhalten von Rapssamen-Cruciferin bei der Chromatofokussierung Cruciferin, ein Hauptspeicherprotein von Rapssamen, ist unter Einsatz der Chromatofokussierung, der isoelektrischen Fokussierung und der Elektrophorese in Polyacrylamidgel untersucht worden. Es wurde gezeigt, daD die Halbmolekiile dieses oligomeren Proteins untereinander hinsichtlich der Zusammensetzung der Untereinheiten differieren. Es gibt spezifische Formen des Cruciferins. Bei niedrigen Ionenstarken lagern sich die Halbmolekiile oder Untereinheiten und/oder Polypeptidketten unter denaturierenden Bedingungen zu relativ stabilen Komplexen zusammen, und zwar offensichtlich durch hydrophobe Wechselwirkungen.
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