E3 ubiquitin (Ub) ligases play diverse roles in cellular regulation in eukaryotes. Three homologous AtRmas (AtRma1, AtRma2, and AtRma3) were recently identified as ER-localized Arabidopsis homologs of human RING membrane-anchor E3 Ub ligase. Here, auxin binding protein 1 (ABP1), one of the auxin receptors in Arabidopsis, was identified as a potential substrate of AtRma2 through a yeast two-hybrid assay. An in vitro pull-down assay confirmed the interaction of full-length AtRma2 with ABP1. AtRma2 was transiently expressed in tobacco (Nicotiana benthamiana) plants through an Agrobacterium-mediated infiltration method and bound ABP1 in vivo. In vitro ubiquitination assays revealed that bacterially-expressed AtRma2 ubiquitinated ABP1. ABP1 was poly-ubiquitinated in tobacco cells and its stability was significantly increased in the presence of MG132, a 26S proteasome inhibitor. This suggests that ABM is controlled by the Ub/26S proteasome system. Therefore, AtRma2 is likely involved in the cellular regulation of ABP1 expression levels. (C) 2010 Elsevier Inc. All rights reserved.