Non-26S Proteasome Endomembrane Trafficking Pathways in ABA Signaling

Non-26S proteasome ubiquitin-dependent degradation pathways are involved in ABA signaling. The endocytic trafficking pathway finely modulates the turnover of ABA regulators, and plays a role in ABA perception, transduction, and action. Autophagy participates in degrading diverse proteins, including both membrane-localized and soluble proteins in the ABA signaling pathway. RING-finger E3 ligases were found to participate in both endosome–vacuole trafficking and in autophagy pathways during ABA signaling. The phytohormone abscisic acid (ABA) is a vital endogenous messenger that regulates diverse physiological processes in plants. The regulation of ABA signaling has been well studied at both the transcriptional and translational levels. Post-translational modification of key regulators in ABA signaling by the 26S ubiquitin proteasome pathway is well known. Recently, increasing evidence demonstrates that atypical turnover of key regulators by the endocytic trafficking pathway and autophagy also play vital roles in ABA perception, signaling, and action. We summarize and synthesize here recent findings in the field of ABA signaling. The phytohormone abscisic acid (ABA) is a vital endogenous messenger that regulates diverse physiological processes in plants. The regulation of ABA signaling has been well studied at both the transcriptional and translational levels. Post-translational modification of key regulators in ABA signaling by the 26S ubiquitin proteasome pathway is well known. Recently, increasing evidence demonstrates that atypical turnover of key regulators by the endocytic trafficking pathway and autophagy also play vital roles in ABA perception, signaling, and action. We summarize and synthesize here recent findings in the field of ABA signaling. a terpenoid phytohormone that plays essential roles in numerous physiological processes, including seed dormancy, germination, plant growth, flowering, and adaptation of plants to various biotic and abiotic stresses. a highly conserved intracellular lysosome/vacuole-dependent catabolic process that mediates degradation of unwanted and damaged constituents such as proteins and organelles [71Papandreou M.E. Tavernarakis N. Autophagy and the endo/exosomal pathways in health and disease.Biotechnol. J. 2016; 12 (Published online December 15, 2016)https://doi.org/10.1002/biot.201600175Crossref PubMed Scopus (44) Google Scholar]. It can be induced by starvation and other adverse conditions. At the initiation of autophagy a double-membraned vesicle, the phagophore, is formed that elongates and seals to generate autophagosomes that enclose autophagic substrates for sequestration. Evidence for possible membrane sources for autophagosome formation in plant cells is limited; however, studies in yeast and animal cells suggest that autophagosomes may originate from multiple organelles, including the endoplasmic reticulum (ER), Golgi, and endosomes. The autophagosome can fuse with lysosomes to form autophagolysosomes, wherein the cargo proteins are degraded by hydrolytic enzymes. Selective autophagy is mediated by different types of autophagic receptors that can specially recognize ubiquitin, particular protein–protein interactions, sugars, or lipid-based autophagic signals [19Khaminets A. et al.Ubiquitin-dependent and independent signals in selective autophagy.Trends Cell Biol. 2016; 26: 6-16Abstract Full Text Full Text PDF PubMed Scopus (472) Google Scholar]. this pathway sorts and delivers both endocytic and biosynthetic proteins. The endocytic pathway is composed of multiple functional compartments that are internalized from the plasma membrane. A portion of the proteins are recycled back to plasma membrane through the recycling endosomes, whereas the remaining proteins are sequentially transported from early endosomes (EEs), late endosomes (LEs), multivesicular bodies (MVBs), and intraluminal vesicles (ILVs) to the lysosome/vacuole for storage or degradation [14Zhuang X.H. et al.Endocytic and autophagic pathways crosstalk in plants.Curr. Opin. Plant Biol. 2015; 28: 39-47Crossref PubMed Scopus (45) Google Scholar]. endomembrane trafficking-related protein-turnover pathways which are independent of the 26S proteasome. These can be ubiquitin-dependent or ubiquitin-independent protein transport and degradation processes, referred to here as endosomal trafficking and autophagy pathways [71Papandreou M.E. Tavernarakis N. Autophagy and the endo/exosomal pathways in health and disease.Biotechnol. J. 2016; 12 (Published online December 15, 2016)https://doi.org/10.1002/biot.201600175Crossref PubMed Scopus (44) Google Scholar]. the 26S proteasome is a multiunit complex that plays a crucial role in ubiquitin-dependent degradation. The 26S proteasome is composed of the 20S core proteasome (CP) and the 19S regulatory particle (RP) [72Smalle J. Vierstra R.D. The ubiquitin 26S proteasome proteolytic pathway.Annu. Rev. Plant Biol. 2004; 55: 555-590Crossref PubMed Scopus (1032) Google Scholar]. The CP is a broad spectrum ATP- and ubiquitin-independent protease that is responsible for digesting proteins into peptides of varying lengths. The RP assists in recognition of ubiquitin conjugates, covalently bound ubiquitin removal, protein unfolding, and directing proteins into the CP for degradation. The 26S proteasome-dependent degradation pathway mainly recognizes and degrades K48-linked polyubiquitin chains, and less frequently targets mono-ubiquitin- and heterotypic K11/K48-linked polyubiquitin-conjugated proteins [73Grice G.L. Nathan J.A. The recognition of ubiquitinated proteins by the proteasome.Cell Mol. Life Sci. 2016; 73: 3497-3506Crossref PubMed Scopus (171) Google Scholar]. the catalytic reaction performed by E1s (ubiquitin activating enzymes), E2s (ubiquitin conjugating enzymes), and E3s (ubiquitin E3 ligases) to produce ubiquitin conjugates. The ubiquitin chains are typically attached to lysine residues of proteins by forming isopeptide bonds between the lysine of the modified protein and the C-terminal glycine of the ubiquitin molecule. Ubiquitination is an important post-translational modification that can change the activity, localization, stability, and structure of the modified proteins.