The ubiquitin-proteasome system regulates plant hormone signaling

Abstract

Plants utilize the ubiquitin-proteasome system (UPS) to modulate nearly every aspect of growth and development. Ubiquitin is covalently attached to target proteins through the action of three enzymes known as E1, E2, and E3. The ultimate outcome of this post-translational modification depends on the nature of the ubiquitin linkage and the extent of polyubiquitination. In most cases, ubiquitination results in degradation of the target protein in the 26S proteasome. During the last 10 years it has become clear that the UPS plays a prominent regulatory role in hormone biology. E3 ubiquitin ligases in particular actively participate in hormone perception, de-repression of hormone signaling pathways, degradation of hormone specific transcription factors, and regulation of hormone biosynthesis. It is certain that additional functions will be discovered as more of the nearly 1200 potential E3s in plants are elucidated.

Figure 1

The ubiquitin-conjugation pathway begins with Ub activation by the E1. Activated Ub is transferred to the E2 ubiquitin-conjugating enzyme. The E3 ubiquitin ligase binds both the protein substrate and E2–Ub to facilitate the transfer of Ub to the substrate. Polyubiquitination often targets a substrate to the 26S proteasome where it is degraded.

Figure 2

E3 Ubiquitin ligases in plants. Single subunit E3 are divided into three classes based on the E2 interaction domains. Substrates are green, substrate specificity factors are purple and the E2 interaction domains are yellow. The HECT-type E3s are unique in that they accept Ub before transferring it to the substrate. Three multi-subunit Es are presented. Cullin is pink, RBX1 is yellow, and the substrate specificity factors are purple. The SCF and CUL4-DDB1 also have adapter subunits that are light green.

Figure 3

Function of E3s in plant hormone biology. (a) Hormone perception. The TIR1 F-box protein binds auxin to promote auxin-regulated transcription through the Ub-mediated degradation of Aux/IAA transcriptional regulators. (b) Signal de-repression. SCF^SLY1 targets DELLA repressor proteins for degradation in response to GA. This action frees the PIF3/4 transcription factors to regulate expression. SCF^TIR1 and SCF^COI1 also act de-repress transcription. (c) Direct regulation of transcription factor levels. SCF^EBF1 acts to keep EIN3 levels low in the absence of ethylene. (d) Regulation of hormone biosynthesis. CUL3-BTB^ETO1 targets type-2 ACC synthases for degradation to limit ethylene production.