title: Mechanism for the degradation of origin recognition complex containing Orc5p with a defective Walker A motif and its suppression by over-production of Orc4p in yeast cells.
abstract: Orc5p is one of six subunits constituting the ORC (origin recognition complex), a possible initiator of chromosomal DNA replication in eukaryotes. Orc5p contains a Walker A motif. We recently reported that a strain of Saccharomyces cerevisiae having a mutation in Orc5p's Walker A motif (orc5-A), showed cell-cycle arrest at G2/M and degradation of ORC at high temperatures (37 degrees C). Over-production of Orc4p, another subunit of ORC, specifically suppressed these phenotypes [Takahashi, Yamaguchi, Yamairi, Makise, Takenaka, Tsuchiya and Mizushima (2004) J. Biol. Chem. 279, 8469-8477]. In the present study, we examined the mechanisms of ORC degradation and of its suppression by Orc4p over-production. In orc5-A, at high temperatures, ORC is degraded by proteasomes; either addition of a proteasome inhibitor, or introduction of a mutation of either tan1-1 or nob1-4 that inhibits proteasomes, prevented ORC degradation. Introduction of the tan1-1 mutation restored cell cycle progression, suggesting that the defect was due to ORC degradation by proteasomes. Yeast two-hybrid and co-immunoprecipitation analyses suggested that Orc5p interacts preferentially with Orc4p and that the orc5-A mutation diminishes this interaction. We suggest that this interaction is mediated by the C-terminal region of Orc4p, and the N-terminal region of Orc5p. Based on these observations, we consider that ATP binding to Orc5p is required for efficient interaction with Orc4p and that, in orc5-A, loss of this interaction at higher temperatures allows proteasomes to degrade ORC, causing growth defects. This model could also explain why over-production of Orc4p suppresses the orc5-A strain's phenotype.
authors.affiliation: Graduate School of Medical and Pharmaceutical Sciences, Kumamoto University, Kumamoto 862-0973, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Makise
authors.forename: Naoko
authors.initials: N
authors.lastname: Takahashi
authors.forename: Kazuya
authors.initials: K
authors.lastname: Matsuda
authors.forename: Fumiko
authors.initials: F
authors.lastname: Yamairi
authors.forename: Keitarou
authors.initials: K
authors.lastname: Suzuki
authors.forename: Tomofusa
authors.initials: T
authors.lastname: Tsuchiya
authors.forename: Tohru
authors.initials: T
authors.lastname: Mizushima
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D020816:Amino Acid Motifs; D001665:Binding Sites; D002453:Cell Cycle; D009154:Mutation / Q000235:genetics; D051738:Origin Recognition Complex / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D061988:Proteasome Inhibitors; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D013696:Temperature; D020798:Two-Hybrid System Techniques
chemical_list: C497219:ORC4 protein, S cerevisiae; C497220:ORC5 protein, S cerevisiae; D051738:Origin Recognition Complex; D061988:Proteasome Inhibitors; D029701:Saccharomyces cerevisiae Proteins; D000255:Adenosine Triphosphate; D046988:Proteasome Endopeptidase Complex