Interaction of the Saccharomyces cerevisiae RING-domain protein Nse1 with Nse3 and the Smc5/6 complex is required for chromosome replication and stability.

Genomic stability is maintained by the concerted actions of numerous protein complexes that participate in chromosomal duplication, repair, and segregation. The Smc5/6 complex is an essential multi-subunit complex crucial for repair of DNA double-strand breaks. Two of its subunits, Nse1 and Nse3, are homologous to the RING-MAGE complexes recently described in human cells. We investigated the contribution of the budding yeast Nse1 RING-domain by isolating a mutant nse1-103 bearing substitutions in conserved Zinc-coordinating residues of the RING-domain that is hypersensitive to genotoxic stress and temperature. The nse1-103 mutant protein was defective in interaction with Nse3 and other Smc5/6 complex subunits, Nse4 and Smc5. Chromosome loss was enhanced, accompanied by a delay in the completion of replication and a modest defect in sister chromatid cohesion, in nse1-103. The nse1-103 mutant was synthetic sick with rrm3∆ (defective in fork passage through pause sites), this defect was rescued by inactivation of Tof1, a subunit of the fork protection complex that enforces pausing. The temperature sensitivity of nse1-103 was partially suppressed by deletion of MPH1, encoding a DNA-helicase. Homology modeling of the structure of the budding yeast Nse1-Nse3 heterodimer based on the human Nse1-MAGEG1 structure suggests a similar organization and indicates that perturbation of the Zn-coordinating cluster has the potential to allosterically alter structural elements at the Nse1/Nse3 interaction interface that may abrogate their association. Our findings demonstrate that the budding yeast Nse1 RING-domain organization is important for interaction with Nse3, which is crucial for completion of chromosomal replication, cohesion, and maintenance of chromosome stability.

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