Why multiple small subunits (Y2 and Y4) for yeast ribonucleotide reductase? Toward understanding the role of Y4

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides. Class I RNRs are composed of two homodimeric subunits: R1 and R2. R1 is directly involved in the reduction, and R2 contains the diferric-tyrosyl radical (Y-.) cofactor essential for the initiation of reduction. Saccharomyces cerevisiae has two RNRs; Y1 and Y3 correspond to R1, whereas Y2 and Y4 correspond to R2. Y4 is essential for diferric-Y-. formation in Y2 from apoY2, Fe2+, and O-2. The actual function of Y4 is controversial, Y2 and Y4 have been further characterized in an effort to understand their respective roles in nucleotide reduction. (HiS)(6)-Y2, Y4, and (HiS)(6)-Y4 are homodimers, isolated largely in apo form. Their CID spectra reveal that they are predominantly helical. The concentrations of Y2 and Y4 in vivo are 0.5-2.3 muM, as determined by Western analysis. Incubation of Y2 and Y4 under physiological conditions generates apo Y2Y4 heterodimer, which can form a diferric-Y-. when incubated with Fe2+ and o(2). Holo Y2Y4 heterodimer contains 0.6-0.8 Y-. and has a specific activity of 0.8-1.3 mu mol(.)min(.)mg. Titration of Y2 with Y4 in the presence of Fe2+ and O-2 gives maximal activity with one equivalent of Y4perY2. Models for the function of Y4 based on these data and the accompanying structure will be discussed.