The role of ubiquitin and the 26S proteasome in plant abiotic stress signaling

Abstract

Ubiquitin is a small, highly conserved, ubiquitously expressed eukaryotic protein with immensely important and diverse regulatory functions. A well-studied function of ubiquitin is its role in selective proteolysis by the ubiquitin-proteasome system (UPS). The UPS has emerged as an integral player in plant response and adaptation to environmental stresses such as drought, salinity, cold and nutrient deprivation. The UPS has also been shown to influence the production and signal transduction of stress-related hormones such as abscisic acid. Understanding UPS function has centered mainly on defining the role of E3 ubiquitin ligases, which are the substrate-recruiting component of the ubiquitination pathway. The recent identification of stress signaling/regulatory proteins that are the subject of ubiquitin-dependent degradation has increased our knowledge of how the UPS facilitates responses to adverse environmental conditions. A brief overview is provided on role of the UPS in modulating protein stability during abiotic stress signaling. E3 ubiquitin ligases for which stress-related substrate proteins have been identified are discussed.

Keywords: 26S proteasome; E3 ubiquitin ligase; abiotic stress; abscisic acid; protein degradation; ubiquitination.

FIGURE 1

Function of E3 ligases in abiotic stress response. (A) Most common type of plant E3s. Ubiquitin ligases are categorized based on the presence of a RING, HECT, or U-box E2-binding domain. RING and U-box domain-containing E3s mediate transfer of ubiquitin (U) directly from the E2-Ub intermediate to the substrate protein. HECT domain-containing E3s form an E3-Ub intermediate prior to the transfer of ubiquitin to the substrate protein. RING domains are found in monomeric E3s and multisubunit CRLs. (B) Illustrations of the most common modes of action for E3 ligases in regulating abiotic stress responses. (1) E3 ligases may function as a negative response regulators required to supress stress response pathways by targeting positive regulators for degradation. (2) E3 ligases may promote stress signaling by functioning as positive response regulators that target negative regulators for degradation following stress perception. (3) E3 ligases may also function to attenuate stress signaling by targeting positive regulators for degradation.

FIGURE 2

Ubiquitin ligases that regulate ABA signaling. Illustration of E3 ligases that regulate ABA synthesis, signal transduction and response. Not all E3 ligases are shown, mainly those with identified substrates. Question marks and dashed lines denote instances where proteasomal-dependent degradation is reported but the E3 ligase involved is unknown.