Plant Science

The precise regulation of the homeostasis of the cellular proteome is critical for the appropriate growth and development of plants. It also allows the plants to respond to various environmental stresses, by modulating their biochemical and physiological aspects in a timely manner. Ubiquitination of cellular proteins is one of the major protein degradation routes for maintaining cellular protein homeostasis, and ubiquitin E3 ligases, components of ubiquitin ligase complexes, play an important role in the selective degradation of target proteins via substrate-specific interactions. Thus, understanding the role of E3 ligases and their substrate regulation uncovers their specific cellular and physiological functions. Here, we provide protocols for auto- and substrate-ubiquitination analyses that utilize the combination of in vitro purified E3 ubiquitin ligase proteins and immunoprecipitation.

Thiol-based redox regulation is a posttranslational protein modification that plays a key role in many biological aspects. To understand its regulatory functions, we need a method to directly assess protein redox state in vivo. Here we present a simple procedure to determine protein redox state in a model plant Arabidopsis thaliana. Our method consists of three key steps: (i) redox fixation by rapidly freezing plant tissues in the liquid nitrogen, (ii) labeling of thiol groups with the maleimide reagent, and (iii) protein detection by Western blotting. The redox state of a specific or given protein can be discriminated by the mobility change on SDS-PAGE with high sensitivity. This method provides a novel strategy to dissect the working dynamics of the redox-regulatory system in plants.

Thiol-disulfide exchange is a key posttranslational modification, determining the folding process of intra- and inter-protein structures. Thiols can be detected by colorimetric reagents, which are stoichiometrically reduced by free thiols, and by fluorescent adducts, showing fluorescence only after thioester formation. We adapted a simple three-step method for detection of disulfide bonds in proteins. After irreversible blocking of protein thiols, disulfide bonds are reduced, followed by the detection of thiols. The approach presented here provides an economical procedure that can be used to obtain a global overview of the thiol-disulfide status of proteins in plants. This method allows the detection of modifications in samples on a gel and can be used for semi-quantitative analysis.

Histone modifications are a group of post-translational modifications on histones which can alter chromatin structure and affect gene expression. Histone ubiquitination is a histone modification found in particular on histone H2A and H2B. Histone ubiquitination can be reversed by ubiquitin-specific proteases (UBP). Here, we describe an in vivo assay for histone deubiquitination activity. After infiltrating UBP12 into Nicotiana benthamiana leaves, H2Aub was visualized by immunocytochemistry. Nicotiana benthamiana leaves, which show high agro infiltration efficiency, were used for transient UBP12 expression for a labor- and time-saving protocol. Reduced H2Aub levels indicated histone deubiquitination activity of UBP12. The clear visualization of nuclei of N. benthamiana leaves makes this method able to easily measure the level of histone modification in vivo by using specific antibodies, providing robust clues of protein function. Thus, this protocol is a powerful complementation to in vitro assays of histone deubiquitination activity.

The process of protein ubiquitination typically consists of three sequential steps to add an ubiquitin (Ub) or Ub chain to a substrate protein, requiring three different enzymes, ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2), and ubiquitin protein ligase (E3). Most E2s possess the classical E2 activity in forming E2-Ub complex through a thioester linkage, in presence of an E1 and Ub. Additionally, some E2s have the ability of catalyzing the formation of free Ub dimer. Such activity indicates an important role of these E2s in ubiquitination pathway. Thus, we developed an in vitro Ub dimer formation assay to determine the activity of certain E2s. Moreover, by using Ub mutants, in which different lysine residues are mutated, the specific linkage of dimer can also be determined.

Protein phosphorylation is one of the most common post-translational modifications in eukaryotic cells and plays a critical role in a vast array of cellular processes. Efficient methods of protein extraction and phosphopeptide purification are required to ensure the detection of high quality of proteins. In our hands, phenol extraction of proteins and TiO₂ chromatography enrich phosphorylated peptides more efficiently than other methods in the moss Physcomitrlla patens (P. patens).

Protein palmitoylation is the post-translational modification of proteins via the attachment of palmitate through acyl linkages. The nucleophile sulfhydryl group of cysteines is the common palmitoylation site. Covalent attachment of palmitate occurs on numerous proteins and is usually associated with directing protein localization to the endomembrane system. Detection of protein palmitoylation by in vivo labeling with tritium-labeled palmitic acid typically requires an autoradiographic exposure time of several months, and, thus is not suitable for rapid analyses. Here, we described an easy protocol for quick in vitro detection of protein S-acylation using the Arabidopsis protein kinase, PBS1, as an example. To determine whether PBS1 is modified through thioester linkage to acyl groups, we employed a “biotin switch” assay (Hemsley et al., 2008). This work was first published in Qi et al. (2014), but we expand on the method here. PBS1 functions within the basal immune system of plants, and is a target of the bacterial cysteine protease, AvrPphB (Shao et al., 2002; Zhang et al., 2010). It contains a predicted N-terminal S-acylation motif (MGCFSCFDS), with both Cys-3 and Cys-6 residues predicted to be palmitoylated by CSS-Palm 3.0 (http://csspalm.biocuckoo.org/; Ren et al., 2008). Our method utilizes hydroxylamine-induced cleavage of thioester bonds, which results in free sulfhydryl groups that can then be conjugated to a biotin derivative, 1-biotinamido-4-[4′-(maleimidomethyl) cyclohexanecarboxamido]-butane (Biotin-BMCC). The conjugates are detectable by Western blot with streptavidin-horseradish peroxidase. The whole process of in vitro labelling and detection took less than 3 days, allowing the fast detection of protein modifications via thioester bonds such as palmitoylation.

Ubiquitin can be added to substrate protein as a protein tag by the concerted actions of ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2) and ubiquitin protein ligase (E3). At the present of E1 and ubiquitin, E2 activity can be determined by the thio-ester formation. The E3 activity of a putative protein as well as the E2/E3 or E3/substrate specificities also can be explored by in vitro ubiquitination assay. The result can be detected by western blot with certain antibody. Purified proteins expressed from bacterial system are always used in this assay.