Function of the ubiquitin-proteosome pathway in auxin response

Proteolysis of important regulatory proteins by the ubiquitin-proteosome pa thway is a key aspect of cellular regulation in eukaryotes. Genetic studies in Arabidopsis indicate that response to auxin depends on the function of proteins in this pathway. The auxin transport inhibitor resistant 1 (TIR1) protein is part of a ubiquitin-protein-ligase complex (E3), known as SKP1 C DC53 F-box(TIR1) (SCFTIR1), that possibly directs ubiquitin-modification of protein regulators of the auxin response. In yeast, a similar E3 complex, SCFCDC4, is regulated by conjugation of the ubiquitin-related protein Rub1 to the Cdc53 protein. In Arabidopsis, the AUXIN-RESISTANT1 (AXR1) gene enco des a subunit of the RUB1-activating enzyme, the first enzyme in the RUB-co njugation pathway. Loss of AXR1 results in loss of auxin response. These re sults suggest a model in which RUB1 modification regulates the activity of SCFTIR1, thereby directing the degradation of the repressors of the auxin r esponse.