DETERMINATION AND MUTATIONAL ANALYSIS OF THE PHOSPHORYLATION SITE IN THE HYPUSINE-CONTAINING PROTEIN HYP2P

Electrospray mass spectrometry of the purified isoforms of the hypusin e-containing protein of Saccharomyces cerevisiae Hyp2p suggested a pho sphorylation of the acidic isoform, which was confirmed by phosphatase treatment. The phosphorylation site was mapped to the N-acetylated se rine residue in position no. 1 by mass spectrometric analysis of enzym atic fragments. Mutation of this serine residue gives rise to only the basic isoform, confirming our protein chemical data. As this mutation has no effect on cell viability or growth rate, the unphosphorylated isoform is sufficient to exert the essential in vivo function of Hyp2p .