Dependence of ORC Silencing Function on NatA-Mediated N^α Acetylation in Saccharomyces cerevisiae

Abstract

N^α acetylation is one of the most abundant protein modifications in eukaryotes and is catalyzed by N-terminal acetyltransferases (NATs). NatA, the major NAT in Saccharomyces cerevisiae, consists of the subunits Nat1p, Ard1p, and Nat5p and is necessary for the assembly of repressive chromatin structures. Here, we found that Orc1p, the large subunit of the origin recognition complex (ORC), required NatA acetylation for its role in telomeric silencing. NatA functioned genetically through the ORC binding site of the HMR-E silencer. Furthermore, tethering Orc1p directly to the silencer circumvented the requirement for NatA in silencing. Orc1p was N^α acetylated in vivo by NatA. Mutations that abrogated its ability to be acetylated caused strong telomeric derepression. Thus, N^α acetylation of Orc1p represents a protein modification that modulates chromatin function in S. cerevisiae. Genetic evidence further supported a functional link between NatA and ORC: (i) nat1Δ was synthetically lethal with orc2-1 and (ii) the synthetic lethality between nat1Δ and SUM1-1 required the Orc1 N terminus. We also found Sir3p to be acetylated by NatA. In summary, we propose a model by which N^α acetylation is required for the binding of silencing factors to the N terminus of Orc1p and Sir3p to recruit heterochromatic factors and establish repression.

We thank A. Brand, S. Bell, J. Berman, J. Boeke, D. Gottschling, E. Jones, J. Rine, S. Rospert, L. Pillus, S. Schaper, D. Shore, and R. Sternglanz for strains and plasmids. We are grateful to U. Marchfelder and A. Barduhn for excellent technical assistance and P. Franke for help with recording the mass spectra. We also thank F. Hucho for his support of this work. We thank M. Shevak for help in preparing the figures; S. Rospert, J. Franke, and S. Schaper for critical reading of the manuscript; and the members of our laboratory for many helpful discussions.

This study was supported by the Deutsche Forschungsgemeinschaft (DFG grant EH 194/1-1 and 1-2) and the Max Planck Society. C.W. also acknowledges support by the Fonds der Chemischen Industrie.