While many ubiquitin-proteasome substrates have been characterized in plants very little is known about the corresponding ubiquitin attachment(s) underlying regulated proteolysis. Current dogma asserts that ubiquitin is typically covalently attached to a substrate via an isopeptide bond between the ubiquitin carboxy-terminus and a substrate lysyl amino group. However non-lysine ubiquitin attachment has been observed in other eukaryotes, including N-terminal, cysteine and serine/threonine modification. Here we investigate site(s) of ubiquitin attachment on IAA1, a short-lived Arabidopsis thaliana Aux/IAA family member. Most Aux/IAA proteins function as negative regulators of auxin responses, and are targeted for degradation after ubiquitination by the ubiquitin ligase SCFTIR1/AFB via an interaction directly facilitated by auxin. Surprisingly, using a HIS6x-HA3x-epitope tagged version expressed in vivo, lysine-less IAA1 was ubiquitinated and rapidly degraded in vivo. Lysine-substituted versions of IAA1 localized to the nucleus as YFP fusions and interacted with both TIR1 and IAA7 in yeast-two-hybrid experiments, indicating that these proteins were functional. Ubiquitination on both HIS6x-HA3x-IAA1 and lysine-less HIS6x-HA3x-IAA1 proteins was sensitive to sodium hydroxide treatment, indicative of ubiquitin oxyester formation on serine or threonine residues. Additionally, base-resistant forms of ubiquitinated IAA1 were observed for HIS6x-HA3x-IAA1, suggesting additional lysyl-linked ubiquitin on this protein. Characterization of other Aux/IAA proteins demonstrated that they have diverse degradation rates, adding further complexity to auxin signaling. Altogether these data indicate that Aux/IAA family members have protein-specific degradation rates and that ubiquitination of Aux/IAAs can occur on multiple types of amino residues to promote rapid, auxin-mediated degradation.
This research output has an Altmetric Attention Score of 3. This is our high-level measure of the quality and quantity of online attention that it has received. This Attention Score, as well as the ranking and number of research outputs shown below, was calculated when the research output was last mentioned on 04 May 2015.
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#16,804,003
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