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Yeast Ribonucleotide Reductase has a Heterodimeric Iron-Radical-Containing Subunit

Andrei Chabes, Vladimir Domkin, Goran Larsson, Aimin Liu, Astrid Graslund, Sybren Wijmenga and Lars Thelander
Proceedings of the National Academy of Sciences of the United States of America
Vol. 97, No. 6 (Mar. 14, 2000), pp. 2474-2479
Stable URL: http://www.jstor.org/stable/122187
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Yeast Ribonucleotide Reductase has a Heterodimeric Iron-Radical-Containing Subunit
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Abstract

Ribonucleotide reductase (RNR) catalyzes the de novo synthesis of deoxyribonucleotides. Eukaryotes have an α 2β 2 form of RNR consisting of two homodimeric subunits, proteins R1 (α 2) and R2 (beta2). The R1 protein is the business end of the enzyme containing the active site and the binding sites for allosteric effectors. The R2 protein is a radical storage device containing an iron center-generated tyrosyl free radical. Previous work has identified an RNR protein in yeast, Rnr4p, which is homologous to other R2 proteins but lacks a number of conserved amino acid residues involved in iron binding. Using highly purified recombinant yeast RNR proteins, we demonstrate that the crucial role of Rnr4p (β ′) is to fold correctly and stabilize the radical-storing Rnr2p by forming a stable 1:1 Rnr2p/Rnr4p complex. This complex sediments at 5.6 S as aβ β ′ heterodimer in a sucrose gradient. In the presence of Rnr1p, both polypeptides of the Rnr2p/Rnr4p heterodimer cosediment at 9.7 S as expected for an α 2β β ′ heterotetramer, where Rnr4p plays an important role in the interaction between the α 2 and the β β ′ subunits. The specific activity of the Rnr2p complexed with Rnr4p is 2,250 nmol deoxycytidine 5′-diphosphate formed per min per mg, whereas the homodimer of Rnr2p shows no activity. This difference in activity may be a consequence of the different conformations of the inactive homodimeric Rnr2p and the active Rnr4p-bound form, as shown by CD spectroscopy. Taken together, our results show that the Rnr2p/Rnr4p heterodimer is the active form of the yeast RNR small subunit.

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