The Role of SUMOylation in the Auxin Response Pathway

Abstract

The Small Ubiquitin-like Modifier 1 (SUMO1) protein is a stress-inducible posttranslational modification present in all eukaryotic organisms. Conjugation of this modifier to a target protein results in the alteration of target protein function. The subsequent de-conjugation of SUMO from target proteins is conducted by a class of enzymes termed SUMO proteases.

Previous research regarding the SUMO E3 ligases SIZ1 and HPY2 has inferred a connection between protein SUMOylation and auxin signalling (Huang, et al., 2009). Here, that connection has been strengthened through phenotypic analysis of Arabidopsis thaliana double knock-out mutant line for the SUMO proteases OVERLY TOLERANT TO SALT 1 and -2 (OTS1 and -2), ots1 ots2, and through the confirmation of the SUMOylation of several auxin cascade proteins.

Loss of OTS1 and -2 was shown to result in an increase in auxin response in Arabidopsis thaliana seedlings exposed to exogenous auxin stimulus, indicating that an increase in global SUMOylation levels alter auxin homeostasis. Further investigation regarding components of the auxin signalling cascade revealed that the auxin receptor, TIR1, and two of the auxin-regulated transcription factors, ARF7 and ARF19, undergo SUMOylation under transient assay conditions. Mutations in TIR1 inducing lysine to arginine substitution of the SUMO-binding residues at each predicted SUMO site eliminated SUMO1 binding under transient assay conditions, further confirming that WT TIR1 is SUMOylated and that the predicted locations were correct. These non-SUMOylatable TIR1 mutant clones were then transformed into the auxin signalling Arabidopsis mutant line tir1/afb2/afb3 to further elucidate the role SUMOylation plays in auxin signalling in planta.