Molecular Evolution by Change of Function

Abstract

Deoxyhypusine synthase participates in the post- translational activation of the eukaryotic initiation fac- tor 5A (eIF5A). The enzyme transfers the aminobutyl moiety of spermidine to a specific lysine residue in the eIF5A precursor protein, i.e. eIF5A(lys). Homospermi- dine synthase catalyzes an analogous reaction but uses putrescine instead of eIF5A(lys) as substrate yielding the rare polyamine homospermidine as product. Homo- spermidine is an essential precursor in the biosynthesis of pyrrolizidine alkaloids, an important class of plant defense compounds against herbivores. Sequence com- parisons of the two enzymes indicate an evolutionary origin of homospermidine synthase from ubiquitous de- oxyhypusine synthase. The two recombinant enzymes from Senecio vernalis were purified, and their proper- ties were compared. Protein-protein binding and ki- netic substrate competition studies confirmed that homospermidine synthase, in comparison to deoxyhy- pusine synthase, lost the ability to bind the eIF5A(lys) to its surface. The two enzymes show the same unique substrate specificities, catalyze the aminobutylation of putrescine with the same specific activities, and exhibit almost identical Michaelis kinetics. In conclusion, ho- mospermidine synthase behaves like a deoxyhypusine synthase that lost its major function (aminobutylation of eIF5A precursor protein) but retained unaltered its side activity (aminobutylation of putrescine). It is sug- gested as having evolved from deoxyhypusine synthase by gene duplication and being recruited for a new function.