THE UBIQUITIN-RELATED PROTEIN RUB1 AND AUXIN RESPONSE IN ARABIDOPSIS

The AXR1 (auxin-resistant) protein, which has features of the ubiquiti n-activating enzyme E1, is required for normal response to the plant h ormone auxin in Arabidopsis thaliana. ECR1 functions together with AXR 1 to activate members of the RUB/NEDD8 family of ubiquitin-related pro teins. Extracts from mutant seedlings lacking AXR1 did not promote for mation of the RUB-ECR1 thiolester, indicating that AXR1 is the major a ctivity in this tissue. AXR1 was localized primarily to the nucleus of dividing and elongating cells, suggesting that the targets of RUB mod ification are nuclear. These results indicate that auxin response depe nds on RUB modification of one or more nuclear proteins.