Yeast homolog of human SAG/ROC2/Rbx2/Hrt2 is essential for cell growth, but not for germination:: chip profiling implicates its role in cell cycle regulation

In an attempt to understand the signaling pathway mediating redox-induced apoptosis, we cloned SAG, an evolutionarily conserved zinc RING finger gene that, when overexpressed, protects cells from apoptosis induced by redox agents. Here we report functional characterization of SAG by the use of yeast genetics approach. Targeted disruption of ySAG, yeast homolog of human SAG, and subsequent tetrad analysis revealed that ySAG is required for yeast viability. Complementation experiment showed that the lethal phenotype induced by the ySAG deletion is fully rescued by wildtype SAG, but not by several hSAG mutants, Complementation experiment has also confirmed that ySAG is essential for normal vegetative growth, rather than being required for sporulation. Furthermore, cell death induced by SAG deletion was accompanied by cell enlargement and abnormal cell cycle profiling with an increased DNA content. Importantly, SAG was found to be the second family member of Rbx (RING box protein) or ROC (Regulator of cullins) or Hrt that is a component of SCF E3 ubiquitin ligase, Indeed, like ROC1/Rbx1/Hrt1, SAG binds to Cull and SAG-Cull complex has ubiquitin ligase activity to promote poly-ubiquitination of E2/ Cdc34. This ligase activity is required for complementation of death phenotype induced by ySAG disruption. Finally, chip profiling of the entire yeast genome revealed induction of several G1/S as well as G2/M checkpoint control genes upon SAG withdrawal. Thus, SAG appears to control cell cycle progression in yeast by promoting ubiquitination and degradation of cell cycle regulatory proteins.