Function of the ubiquitin-proteosome pathway in auxin response.

Proteolysis of important regulatory proteins by the ubiquitin-proteosome pathway 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 CDC53 F-boxTIR1 (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 encodes a subunit of the RUB1-activating enzyme, the first enzyme in the RUB-conjugation pathway. Loss of AXR1 results in loss of auxin response. These results suggest a model in which RUB1 modification regulates the activity of SCFTIR1, thereby directing the degradation of the repressors of the auxin response.