Regulation of the activity of ORC.

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Orc5p is one of six subunits constituting the origin recognition complex (ORC), 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°C). Over-production of Orc4p, another subunit of ORC, specifically suppressed these phenotypes. In this 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 tanl-1 or nob1-4 that inhibits proteasomes, prevented ORC degradation. Introduction of the tanl-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 om5-A strain's phenotype.