The auxin-binding pocket of auxin-binding protein 1 comprises the highly conserved boxes a and c

The Arabidopsis cell cycle F-box protein SKP2A binds to auxin

Arabidopsis thaliana S-Phase Kinase-Associated Protein 2A (SKP2A) is an F-box protein that regulates the proteolysis of cell cycle transcription factors. The plant hormone auxin regulates multiple aspects of plant growth and development, including cell division. We found that auxin induces the ubiquitin-dependent degradation of SKP2A both in vivo and in vitro, suggesting that this hormone acts as a signal to trigger SKP2A proteolysis. In this article, we show that auxin binds directly and specifically to SKP2A. By TIR1-based superposition and docking analyzes, we identified an auxin binding site in SKP2A. Mutations in this binding site reduce the ability of SKP2A to bind to auxin and generate nondegradable SKP2A forms. In addition, these non-auxin binding proteins are unable to promote E2FC/DPB degradation in vivo or to induce cell division in the root meristem. Auxin binds to TIR1 to promote its interaction with the auxin/indole-3-acetic acid target proteins. Here, we show that auxin also enhanced the interaction between SKP2A and DPB. Finally, a mutation in SKP2A leads to auxin-resistant root growth, an effect that is additive with the tir1-1 phenotype. Thus, our data indicate that SKP2A is an auxin binding protein that connects auxin signaling with cell division.

ABP1: an auxin receptor for fast responses at the plasma membrane

Auxin-binding protein 1 (ABP1) is an auxin receptor for responses not primarily regulated by gene regulation. One fast response is protoplast swelling. By using immunological ABP1 tools we showed that the highly conserved box a is not alone important for auxin binding. Box c is another part of the auxin binding domain.1 Here we present a novel method to analyze auxin-induced, ABP1-mediated effects at the plasma membrane on single cell level in vivo. The fluorescence of FM4-64 in the plasma membrane is reduced by auxin and this response is mediated by ABP1. This method indicates a functional role of ABP1 at the plasma membrane.