| The AXR1 gene of Arabidopsis thaliana encodes a protein with sequence similarity to the N-terminal half of the ubiquitin activating enzyme "E1". AXR1 was the first gene encoding a protein of this structure to be cloned. In order to better model the role of this gene in cell physiology, a genetic analysis of two AXR1 homologues was undertaken in the genetically tractable ascomycete fungus Saccharomyces cereviciae. Through genetic analysis, one of these AXR1 homologues, designated ENR2, was found to function in the biochemical pathway which effects the passage of S. cereviciae cells from the G1 phase of the cell cycle to the S phase of the cell cycle. The G1 to S transition of the cell cycle is brought about through the action of a multisubunit complex designated SCFCdc4. Biochemical analysis of SCFCdc4 in enr2 mutant backgrounds revealed that the physical state of one component of SCFCdc4, Cdc53p, was dependent on the action of ENR2. Cdc53p is normally present as two isoforms of discrete molecular weights. In enr2 mutant backgrounds, only the lower molecular weight isoform is present. The higher molecular weight isoform of Cdc53p was found to be a post-translationally modified form of native Cdc53p, formed through the conjugation of Cdc53p to a second protein designated Rub1p. Rub1p is a homologue of the ubiquitin protein. These results suggest a model for the activity of the AXR1 family in which these proteins function in the activation step of the biochemical pathways leading to conjugation of ubiquitin homologues to various substrate proteins. |
