Histone deacetylases: salesmen and customers in the post‐translational modification market

HDACs (histone deacetylases) are enzymes that remove the acetyl moiety from N‐?‐acetylated lysine residues in histones and non‐histone proteins. In recent years, it has turned out that HDACs themselves are also subject to post‐translational modification. Such structural alterations can determine the stability, localization, activity and protein—protein interactions of HDACs. This subsequently affects the modification of their substrates and the co‐ordination of cellular signalling networks. Intriguingly, physiologically relevant non‐histone proteins are increasingly found to be deacetylated by HDACs, and aberrant deacetylase activity contributes to several severe human diseases. Targeting the catalytic activity of these enzymes and their post‐translational modifications are therefore attractive targets for therapeutical intervention strategies. To achieve this ambitious goal, details on the molecular mechanisms regulating post‐translational modifications of HDACs are required. This review summarizes aspects of the current knowledge on the biological role and enzymology of the phosphorylation, acetylation, ubiquitylation and sumoylation of HDACs.     

Accumulation and post‐translational modifications of plant tubulins

The microtubular cytoskeleton of plant cells provides support for several functions (including the anchoring of proteins, assembly of the mitotic spindle, cytoplasmic streaming and construction of cell walls). Both α‐ and β‐tubulins are encoded through multigene families that are differentially expressed in different organs and tissues. To increase the variability of expression, both protein subunits are subjected to post‐translational modifications, which could contribute to the assembly of specific microtubule structures. This review aims to highlight the role of specific post‐translational modifications of tubulin in plant cells. We initially describe the expression and accumulation of α‐ and β‐tubulin isoforms in different plants and at different stages of plant development. Second, we discuss the different types of post‐translational modifications that, by adding or removing specific functional groups, increase the isoform heterogeneity and functional variability of tubulin. Modifications are proposed to form a ‘code’ that can be read by proteins interacting with microtubules. Therefore, the subpopulations of microtubules may bind to different associated proteins (motor and non‐motor), thus creating the physical support for various microtubule functions.     

SUPPRESSOR OF LLP1 1‐mediated C–terminal processing is critical for CLE19 peptide activity

Cell‐to‐cell communication is essential for the coordinated development of multicellular organisms. Members of the CLAVATA3/EMBRYO‐SURROUNDING REGION‐RELATED (CLE) family, a group of small secretory peptides, are involved in these processes in plants. Although post‐translational modifications are considered to be indispensable for their activity, the detailed mechanisms governing these modifications are not well understood. Here, we report that SUPPRESSOR OF LLP1 1 (SOL1), a putative Zn²⁺ carboxypeptidase previously isolated as a suppressor of the CLE19 over‐expression phenotype, functions in C–terminal processing of the CLE19 proprotein to produce the functional CLE19 peptide. Newly isolated sol1 mutants are resistant to CLE19 over‐expression, consistent with the previous report (Casamitjana‐Martinez, E., Hofhuis, H.F., Xu, J., Liu, C.M., Heidstra, R. and Scheres, B. (2003) Curr. Biol. 13, 1435–1441). As expected, our experiment using synthetic CLE19 peptide revealed that the sol1 mutation does not compromise CLE signal transduction pathways per se. SOL1 possesses enzymatic activity to remove the C–terminal arginine residue of CLE19 proprotein in vitro, and SOL1‐dependent cleavage of the C–terminal arginine residue is necessary for CLE19 activity in vivo. Additionally, the endosomal localization of SOL1 suggests that this processing occurs in endosomes in the secretory pathway. Thus, our data indicate the importance of C–terminal processing of CLE proproteins to ensure CLE activities.     

Functional analysis tools for post‐translational modification: a post‐translational modification database for analysis of proteins and metabolic pathways

Post‐translational modifications (PTMs) are critical regulators of protein function, and nearly 200 different types of PTM have been identified. Advances in high‐resolution mass spectrometry have led to the identification of an unprecedented number of PTM sites in numerous organisms, potentially facilitating a more complete understanding of how PTMs regulate cellular behavior. While databases have been created to house the resulting data, most of these resources focus on individual types of PTM, do not consider quantitative PTM analyses or do not provide tools for the visualization and analysis of PTM data. Here, we describe the Functional Analysis Tools for Post‐Translational Modifications (FAT‐PTM) database ( https://bioinformatics.cse.unr.edu/fat-ptm/ ), which currently supports eight different types of PTM and over 49 000 PTM sites identified in large‐scale proteomic surveys of the model organism Arabidopsis thaliana. The FAT‐PTM database currently supports tools to visualize protein‐centric PTM networks, quantitative phosphorylation site data from over 10 different quantitative phosphoproteomic studies, PTM information displayed in protein‐centric metabolic pathways and groups of proteins that are co‐modified by multiple PTMs. Overall, the FAT‐PTM database provides users with a robust platform to share and visualize experimentally supported PTM data, develop hypotheses related to target proteins or identify emergent patterns in PTM data for signaling and metabolic pathways.     

Regulation of connexin‐ and pannexin‐based channels by post‐translational modifications

Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi‐protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post‐translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N‐acetylation, S‐nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx‐channel activity and localisation.     

The ER luminal C‐terminus of AtSec62 is critical for male fertility and plant growth in Arabidopsis thaliana

Protein translocation into the endoplasmic reticulum (ER) occurs either co‐ or post‐translationally through the Sec translocation system. The Arabidopsis Sec post‐translocon is composed of the protein‐conducting Sec61 complex, the chaperone‐docking protein AtTPR7, the J‐domain‐containing proteins AtERdj2A/B and the yet uncharacterized AtSec62. Yeast Sec62p is suggested to mainly function in post‐translational translocation, whereas mammalian Sec62 also interacts with ribosomes. In Arabidopsis, loss of AtSec62 leads to impaired growth and drastically reduced male fertility indicating the importance of AtSec62 in protein translocation and subsequent secretion in male gametophyte development. Moreover, AtSec62 seems to be divergent in function as compared with yeast Sec62p, since we were not able to complement the thermosensitive yeast mutant sec62‐ts. Interestingly, AtSec62 has an additional third transmembrane domain in contrast to its yeast and mammalian counterparts resulting in an altered topology with the C‐terminus facing the ER lumen instead of the cytosol. In addition, the AtSec62 C‐terminus has proven to be indispensable for AtSec62 function, since a construct lacking the C‐terminal region was not able to rescue the mutant phenotype in Arabidopsis. We thus propose that Sec62 acquired a unique topology and function in protein translocation into the ER in plants.     

Arabidopsis plants expressing only the redox‐regulated Rca‐α isoform have constrained photosynthesis and plant growth

Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors from the active sites of Rubisco and thereby plays a central role in initiating and sustaining Rubisco activation. In Arabidopsis, alternative splicing of a single Rca gene results in two Rca isoforms, Rca‐α and Rca‐β. Redox modulation of Rca‐α regulates the function of Rca‐α and Rca‐β acting together to control Rubisco activation. Although Arabidopsis Rca‐α alone less effectively activates Rubisco in vitro, it is not known how CO₂ assimilation and plant growth are impacted. Here, we show that two independent transgenic Arabidopsis lines expressing Rca‐α in the absence of Rca‐β (‘Rca‐α only’ lines) grew more slowly in various light conditions, especially under low light or fluctuating light intensity, and in a short day photoperiod compared to wildtype. Photosynthetic induction was slower in the Rca‐α only lines, and they maintained a lower rate of CO₂ assimilation during both photoperiod types. Our findings suggest Rca oligomers composed of Rca‐α only are less effective in initiating and sustaining the activation of Rubisco than when Rca‐β is also present. Currently there are no examples of any plant species that naturally express Rca‐α only but numerous examples of species expressing Rca‐β only. That Rca‐α exists in most plant species, including many C3 and C4 food and bioenergy crops, implies its presence is adaptive under some circumstances.     

Phytotyping4D: a light‐field imaging system for non‐invasive and accurate monitoring of spatio‐temporal plant growth

Integrative studies of plant growth require spatially and temporally resolved information from high‐throughput imaging systems. However, analysis and interpretation of conventional two‐dimensional images is complicated by the three‐dimensional nature of shoot architecture and by changes in leaf position over time, termed hyponasty. To solve this problem, Phytotyping^4D uses a light‐field camera that simultaneously provides a focus image and a depth image, which contains distance information about the object surface. Our automated pipeline segments the focus images, integrates depth information to reconstruct the three‐dimensional architecture, and analyses time series to provide information about the relative expansion rate, the timing of leaf appearance, hyponastic movement, and shape for individual leaves and the whole rosette. Phytotyping^4D was calibrated and validated using discs of known sizes, and plants tilted at various orientations. Information from this analysis was integrated into the pipeline to allow error assessment during routine operation. To illustrate the utility of Phytotyping^4D, we compare diurnal changes in Arabidopsis thaliana wild‐type Col‐0 and the starchless pgm mutant. Compared to Col‐0, pgm showed very low relative expansion rate in the second half of the night, a transiently increased relative expansion rate at the onset of light period, and smaller hyponastic movement including delayed movement after dusk, both at the level of the rosette and individual leaves. Our study introduces light‐field camera systems as a tool to accurately measure morphological and growth‐related features in plants.     

Conserved V‐ATPase c subunit plays a role in plant growth by influencing V‐ATPase‐dependent endosomal trafficking

In plant cells, the vacuolar‐type H⁺‐ATPases (V‐ATPase) are localized in the tonoplast, Golgi, trans‐Golgi network and endosome. However, little is known about how V‐ATPase influences plant growth, particularly with regard to the V‐ATPase c subunit (VHA‐c). Here, we characterized the function of a VHA‐c gene from Puccinellia tenuiflora (PutVHA‐c) in plant growth. Compared to the wild‐type, transgenic plants overexpressing PutVHA‐c in Arabidopsis thaliana exhibit better growth phenotypes in root length, fresh weight, plant height and silique number under the normal and salt stress conditions due to noticeably higher V‐ATPase activity. Consistently, the Arabidopsis atvha‐c5 mutant shows reduced V‐ATPase activity and retarded plant growth. Furthermore, confocal and immunogold electron microscopy assays demonstrate that PutVHA‐c is mainly localized to endosomal compartments. The treatment of concanamycin A (ConcA), a specific inhibitor of V‐ATPases, leads to obvious aggregation of the endosomal compartments labelled with PutVHA‐c‐GFP. Moreover, ConcA treatment results in the abnormal localization of two plasma membrane (PM) marker proteins Pinformed 1 (AtPIN1) and regulator of G protein signalling‐1 (AtRGS1). These findings suggest that the decrease in V‐ATPase activity blocks endosomal trafficking. Taken together, our results strongly suggest that the PutVHA‐c plays an important role in plant growth by influencing V‐ATPase‐dependent endosomal trafficking.     

Hydroxyproline O‐arabinosyltransferase mutants oppositely alter tip growth in Arabidopsis thaliana and Physcomitrella patens

Hydroxyproline O‐arabinosyltransferases (HPATs) are members of a small, deeply conserved family of plant‐specific glycosyltransferases that add arabinose sugars to diverse proteins including cell wall‐associated extensins and small signaling peptides. Recent genetic studies in flowering plants suggest that different HPAT homologs have been co‐opted to function in diverse species‐specific developmental contexts. However, nothing is known about the roles of HPATs in basal plants. We show that complete loss of HPAT function in Arabidopsis thaliana and the moss Physcomitrella patens results in a shared defect in gametophytic tip cell growth. Arabidopsis hpat1/2/3 triple knockout mutants suffer from a strong male sterility defect as a consequence of pollen tubes that fail to fully elongate following pollination. Knocking out the two HPAT genes of Physcomitrella results in larger multicellular filamentous networks due to increased elongation of protonemal tip cells. Physcomitrella hpat mutants lack cell‐wall associated hydroxyproline arabinosides and can be rescued with exogenous cellulose, while global expression profiling shows that cell wall‐associated genes are severely misexpressed, implicating a defect in cell wall formation during tip growth. Our findings point to a major role for HPATs in influencing cell elongation during tip growth in plants.     

Computational identification of post‐translational modification‐based nuclear import regulations by characterizing nuclear localization signal‐import receptor interaction

The binding affinity between a nuclear localization signal (NLS) and its import receptor is closely related to corresponding nuclear import activity. PTM‐based modulation of the NLS binding affinity to the import receptor is one of the most understood mechanisms to regulate nuclear import of proteins. However, identification of such regulation mechanisms is challenging due to the difficulty of assessing the impact of PTM on corresponding nuclear import activities. In this study we proposed NIpredict, an effective algorithm to predict nuclear import activity given its NLS, in which molecular interaction energy components (MIECs) were used to characterize the NLS‐import receptor interaction, and the support vector regression machine (SVR) was used to learn the relationship between the characterized NLS‐import receptor interaction and the corresponding nuclear import activity. Our experiments showed that nuclear import activity change due to NLS change could be accurately predicted by the NIpredict algorithm. Based on NIpredict, we developed a systematic framework to identify potential PTM‐based nuclear import regulations for human and yeast nuclear proteins. Application of this approach has identified the potential nuclear import regulation mechanisms by phosphorylation of two nuclear proteins including SF1 and ORC6. Proteins 2014; 82:2783–2796. © 2014 Wiley Periodicals, Inc.     

PP7L is essential for MAIL1‐mediated transposable element silencing and primary root growth

The two paralogous Arabidopsis genes MAINTENANCE OF MERISTEMS (MAIN) and MAINTENANCE OF MERISTEMS LIKE1 (MAIL1) encode a conserved retrotransposon‐related plant mobile domain and are known to be required for silencing of transposable elements (TE) and for primary root development. Loss of function of either MAIN or MAIL1 leads to release of heterochromatic TEs, reduced condensation of pericentromeric heterochromatin, cell death of meristem cells and growth arrest of the primary root soon after germination. Here, we show that they act in one protein complex that also contains the inactive isoform of PROTEIN PHOSPHATASE 7 (PP7), which is named PROTEIN PHOSPHATASE 7‐LIKE (PP7L). PP7L was previously shown to be important for chloroplast biogenesis and efficient chloroplast protein synthesis. We show that loss of PP7L function leads to the same root growth phenotype as loss of MAIL1 or MAIN. In addition, pp7l mutants show similar silencing defects. Double mutant analyses confirmed that the three proteins act in the same molecular pathway. The primary root growth arrest, which is associated with cell death of stem cells and their daughter cells, is a consequence of genome instability. Our data demonstrate so far unrecognized functions of an inactive phosphatase isoform in a protein complex that is essential for silencing of heterochromatic elements and for maintenance of genome stability in dividing cells.     

Multiple cyclic nucleotide‐gated channels coordinate calcium oscillations and polar growth of root hairs

Calcium gradients underlie polarization in eukaryotic cells. In plants, a tip‐focused Ca²⁺‐gradient is fundamental for rapid and unidirectional cell expansion during epidermal root hair development. Here we report that three members of the cyclic nucleotide‐gated channel family are required to maintain cytosolic Ca²⁺ oscillations and the normal growth of root hairs. CNGC6, CNGC9 and CNGC14 were expressed in root hairs, with CNGC9 displaying the highest root hair specificity. In individual channel mutants, morphological defects including root hair swelling and branching, as well as bursting, were observed. The developmental phenotypes were amplified in the three cngc double mutant combinations. Finally, cngc6/9/14 triple mutants only developed bulging trichoblasts and could not form normal root hair protrusions because they burst after the transition to the rapid growth phase. Prior to developmental defects, single and double mutants showed increasingly disturbed patterns of Ca²⁺ oscillations. We conclude that CNGC6, CNGC9 and CNGC14 fulfill partially but not fully redundant functions in generating and maintaining tip‐focused Ca²⁺ oscillations, which are fundamental for proper root hair growth and polarity. Furthermore, the results suggest that these calmodulin‐binding and Ca²⁺‐permeable channels organize a robust tip‐focused oscillatory calcium gradient, which is not essential for root hair initiation but is required to control the integrity of the root hair after the transition to the rapid growth phase. Our findings also show that root hairs possess a large ability to compensate calcium‐signaling defects, and add new players to the regulatory network, which coordinates cell wall properties and cell expansion during polar root hair growth.     

Tissue‐specific inactivation by cytosine deaminase/uracil phosphoribosyl transferase as a tool to study plant biology

Recent advances in the study of plant developmental and physiological responses have benefited from tissue‐specific approaches, revealing the role of some cell types in these processes. Such approaches have relied on the inactivation of target cells using either toxic compounds or deleterious genes; however, both tissue‐specific and truly inducible tools are lacking in order to precisely target a developmental window or specific growth response. We engineered the yeast fluorocytosine deaminase (FCY1) gene by creating a fusion with the bacterial uracil phosphoribosyl transferase (UPP) gene. The recombinant protein converts the precursor 5‐fluorocytosine (5‐FC) into 5‐fluorouracyl, a drug used in the treatment of a range of cancers, which triggers DNA and RNA damage. We expressed the FCY‐UPP gene construct in specific cell types using enhancer trap lines and promoters, demonstrating that this marker acts in a cell‐autonomous manner. We also showed that it can inactivate slow developmental processes like lateral root formation by targeting pericycle cells. It also revealed a role for the lateral root cap and the epidermis in controlling root growth, a faster response. The 5‐FC precursor acts systemically, as demonstrated by its ability to inhibit stomatal movements when supplied to the roots in combination with a guard cell‐specific promoter. Finally, we demonstrate that the tissular inactivation is reversible, and can therefore be used to synchronize plant responses or to determine cell type‐specific functions during different developmental stages. This tool will greatly enhance our capacity to understand the respective role of each cell type in plant physiology and development.