Structural insight into the reaction mechanism and evolution of cytokinin biosynthesis.
The phytohormone cytokinin regulates plant growth and development. This hormone is also synthesized by some phytopathogenic bacteria, such as Agrobacterium tumefaciens, and is as a key factor in the formation of plant tumors. The rate-limiting step of cytokinin biosynthesis is catalyzed by adenosine phosphate-isopentenyltransferase (IPT). Agrobacterium IPT has a unique substrate specificity that enables it to increase trans-zeatin production by recruiting a metabolic intermediate of the host plant's biosynthetic pathway. Here, we show the crystal structures of Tzs, an IPT from A. tumefaciens, complexed with AMP and a prenyl-donor analogue, dimethylallyl S-thiodiphosphate. The structures reveal that the carbon-nitrogen-based prenylation proceeds by the SN2-reaction mechanism. Site-directed mutagenesis was used to determine the amino acid residues, Asp-173 and His-214, which are responsible for differences in prenyl-donor substrate specificity between plant and bacterial IPTs. IPT and the p loop-containing nucleoside triphosphate hydrolases likely evolved from a common ancestral protein. Despite structural similarities, IPT has evolved a distinct role in which the p loop transfers a prenyl moiety in cytokinin biosynthesis.
Sugawara
Hajime
H
RIKEN Plant Science Center, 1-7-22 Suehiro, Tsurumi, Yokohama 230-0045, Japan.
Ueda
Nanae
N
Kojima
Mikiko
M
Makita
Nobue
N
Yamaya
Tomoyuki
T
Sakakibara
Hitoshi
H
D000249:Adenosine Monophosphate / Q000737:chemistry / Q000378:metabolism; D016960:Agrobacterium tumefaciens / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000596:Amino Acids / Q000737:chemistry / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D018360:Crystallography, X-Ray; D003583:Cytokinins / Q000096:biosynthesis* / Q000737:chemistry; D019143:Evolution, Molecular; D007700:Kinetics; D008958:Models, Molecular; D009946:Organothiophosphorus Compounds / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D013379:Substrate Specificity; D015032:Zinc / Q000737:chemistry / Q000378:metabolism
D000596:Amino Acids; D001426:Bacterial Proteins; D003583:Cytokinins; D009946:Organothiophosphorus Compounds; C486057:dimethyl thiophosphite; D000249:Adenosine Monophosphate; D015032:Zinc