MAP kinase phosphorylation of plant profilin

Revisiting the role of MAPK signalling pathway in plants and its manipulation for crop improvement

The mitogen-activated protein kinase (MAPK) pathway is an important signalling event associated with every aspect of plant growth, development, yield, abiotic and biotic stress adaptation. Being a central metabolic pathway, it is a vital target for manipulation for crop improvement. In this review, we have summarised recent advancements in understanding involvement of MAPK signalling in modulating abiotic and biotic stress tolerance, architecture and yield of plants. MAPK signalling cross talks with reactive oxygen species (ROS) and abscisic acid (ABA) signalling events in bringing about abiotic stress adaptation in plants. The intricate involvement of MAPK pathway with plant's pathogen defence ability has also been identified. Further, recent research findings point towards participation of MAPK signalling in shaping plant architecture and yield. These make MAPK pathway an important target for crop improvement and we discuss here various strategies to tweak MAPK signalling components for designing future crops with improved physiology and phenotypes.

Structural insights into the amino acid usage variations in the profilin gene family

Profilin protein is present ubiquitously in all forms of life and is allied with allergic responses among atopic individuals. In addition to this, profilins from various food sources are also associated with IgE cross-reactivity and are thus classified as pan-allergens. The present study unravels the physicochemical basis of differential amino acid usage patterns observed in the profilin gene family. Correspondence analysis based on amino acid usage of allergen and non-allergen profilins revealed discrete clusters among them, signifying differential patterns of amino acid usage. The amino acids, namely methionine, proline, histidine, glutamine, glutamic acid, tryptophan and glycine were found to be more frequently utilised by the allergen profilins compared to the non-allergens. Correlation analysis revealed that physicochemical features like protein disorder, trypsin digestion and solubility differed significantly among the allergen and non-allergen profilins, thus supporting the observations from correspondence analysis. In addition, comprehensive sequence analysis revealed that the allergen profilins possess conserved motifs which may correlate with their distinct physicochemical features. An in-depth structural analysis revealed that the over-represented amino acids in allergen profilins have a propensity of being exposed on the surface, which may be attributed to their distinct allergenic characteristics. The distinguished physicochemical features observed among allergens and non-allergens can be employed as descriptors to develop machine learning-based allergenicity prediction models.

Are Physicochemical Properties Shaping the Allergenic Potency of Plant Allergens?

This review searched for published evidence that could explain how different physicochemical properties impact on the allergenicity of food proteins and if their effects would follow specific patterns among distinct protein families. Owing to the amount and complexity of the collected information, this literature overview was divided in two articles, the current one dedicated to protein families of plant allergens and a second one focused on animal allergens. Our extensive analysis of the available literature revealed that physicochemical characteristics had consistent effects on protein allergenicity for allergens belonging to the same protein family. For example, protein aggregation contributes to increased allergenicity of 2S albumins, while for legumins and cereal prolamins, the same phenomenon leads to a reduction. Molecular stability, related to structural resistance to heat and proteolysis, was identified as the most common feature promoting plant protein allergenicity, although it fails to explain the potency of some unstable allergens (e.g. pollen-related food allergens). Furthermore, data on physicochemical characteristics translating into clinical effects are limited, mainly because most studies are focused on in vitro IgE binding. Clinical data assessing how these parameters affect the development and clinical manifestation of allergies is minimal, with only few reports evaluating the sensitising capacity of modified proteins (addressing different physicochemical properties) in murine allergy models. In vivo testing of modified pure proteins by SPT or DBPCFC is scarce. At this stage, a systematic approach to link the physicochemical properties with clinical plant allergenicity in real-life scenarios is still missing.

The Quest for MAP Kinase Substrates: Gaining Momentum

Mitogen-activated protein kinase (MAPK) pathways are versatile signaling mechanisms in all eukaryotes. Their signaling outputs are defined by the protein substrates phosphorylated by MAPKs. An expanding list of substrates has been identified by high-throughput screens and targeted approaches in plants. The majority of these are phosphorylated by MPK3/6, and a few by MPK4, which are the best-characterized plant MAPKs, participating in the regulation of numerous biological processes. The identified substrates clearly represent the functional diversity of MAPKs: they are associated with pathogen defense, abiotic stress responses, ethylene signaling, and various developmental functions. Understanding their outputs is integral to unraveling the complex regulatory mechanisms of MAPK cascades. We review here methodological approaches and provide an overview of known MAPK substrates.

Cell and Developmental Biology of Plant Mitogen-Activated Protein Kinases

Plant mitogen-activated protein kinases (MAPKs) constitute a network of signaling cascades responsible for transducing extracellular stimuli and decoding them to dedicated cellular and developmental responses that shape the plant body. Over the last decade, we have accumulated information about how MAPK modules control the development of reproductive tissues and gametes and the embryogenic and postembryonic development of vegetative organs such as roots, root nodules, shoots, and leaves. Of key importance to understanding how MAPKs participate in developmental and environmental signaling is the characterization of their subcellular localization, their interactions with upstream signal perception mechanisms, and the means by which they target their substrates. In this review, we summarize the roles of MAPK signaling in the regulation of key plant developmental processes, and we survey what is known about the mechanisms guiding the subcellular compartmentalization of MAPK modules.

Histone H2B monoubiquitination regulates salt stress-induced microtubule depolymerization in Arabidopsis

Histone H2B monoubiquitination (H2Bub1) is recognized as a regulatory mechanism that controls a range of cellular processes. We previously showed that H2Bub1 was involved in responses to biotic stress in Arabidopsis. However, the molecular regulatory mechanisms of H2Bub1 in controlling responses to abiotic stress remain limited. Here, we report that HISTONE MONOUBIQUITINATION1 (HUB1) and HUB2 played important regulatory roles in response to salt stress. Phenotypic analysis revealed that H2Bub1 mutants confer decreased tolerance to salt stress. Further analysis showed that H2Bub1 regulated the depolymerization of microtubules (MTs), the expression of PROTEIN TYROSINE PHOSPHATASE1 (PTP1) and MAP KINASE PHOSPHATASE (MKP) genes - DsPTP1, MKP1, IBR5, PHS1, and was required for the activation of mitogen-activated protein kinase3 (MAP kinase3, MPK3) and MPK6 in response to salt stress. Moreover, both tyrosine phosphorylation and the activation of MPK3 and MPK6 affected MT stability in salt stress response. Thus, the results indicate that H2Bub1 regulates salt stress-induced MT depolymerization, and the PTP-MPK3/6 signalling module is responsible for integrating signalling pathways that regulate MT stability, which is critical for plant salt stress tolerance.

Evolutionary expansion and structural functionalism of the ancient family of profilin proteins

Structure and functional similarities of a recent protein's orthologs with its ancient counterpart are largely determined by the configuration of evolutionary preservation of amino acids. The emergence of genome sequencing databases allowed dissecting the evolutionarily important gene families at a comprehensive and genome-wide scale. The profilin multi-gene family is an ancient, universal, and functionally diverged across kingdoms, which regulates various aspects of cellular development in both prokarya and eukarya, especially cell-wall maintenance through actin sequestering, nucleation and cytokinesis. We performed a meta-analysis of the evolutionary expansion, structural conservation, evolution of function motifs, and transcriptional biases of profilin proteins across kingdoms. An exhaustive search of various genome databases of cyanobacteria, fungi, animalia and plantae kingdoms revealed 172 paralogous/orthologous profilins those were phylogenetically clustered in various groups. Orthologous gene comparisons indicated that segmental and tandem duplication events under strong purifying selection are predominantly responsible for their convoluted structural divergences. Evidently, structural divergences were more prevalent in the paralogs than orthologs, and evolutionary variations in the exon/intron architecture were accomplished by 'exon/intron-gain' and insertion/deletion during sequence-exonization. Remarkably, temporal expression evolution of profilin paralogs/homeologs during cotton fiber domestication provides evolutionary impressions of the selection of highly diverged transcript abundance notably in the fiber morpho-evolution. These results provide global insights into the profilin evolution, their structural design across taxa; and their future utilization in translational research.

Timing is everything: highly specific and transient expression of a MAP kinase determines auxin-induced leaf venation patterns in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules present in all eukaryotes. In plants, MAPK cascades were shown to regulate cell division, developmental processes, stress responses, and hormone pathways. The subgroup A of Arabidopsis MAPKs consists of AtMPK3, AtMPK6, and AtMPK10. AtMPK3 and AtMPK6 are activated by their upstream MAP kinase kinases (MKKs) AtMKK4 and AtMKK5 in response to biotic and abiotic stress. In addition, they were identified as key regulators of stomatal development and patterning. AtMPK10 has long been considered as a pseudo-gene, derived from a gene duplication of AtMPK6. Here we show that AtMPK10 is expressed highly but very transiently in seedlings and at sites of local auxin maxima leaves. MPK10 encodes a functional kinase and interacts with the upstream MAP kinase kinase (MAPKK) AtMKK2. mpk10 mutants are delayed in flowering in long-day conditions and in continuous light. Moreover, cotyledons of mpk10 and mkk2 mutants have reduced vein complexity, which can be reversed by inhibiting polar auxin transport (PAT). Auxin does not affect AtMPK10 expression while treatment with the PAT inhibitor HFCA extends the expression in leaves and reverses the mpk10 mutant phenotype. These results suggest that the AtMKK2-AtMPK10 MAPK module regulates venation complexity by altering PAT efficiency.

Analysis of the effects of polymorphism on pollen profilin structural functionality and the generation of conformational, T- and B-cell epitopes

An extensive polymorphism analysis of pollen profilin, a fundamental regulator of the actin cytoskeleton dynamics, has been performed with a major focus in 3D-folding maintenance, changes in the 2-D structural elements, surface residues involved in ligands-profilin interactions and functionality, and the generation of conformational and lineal B- and T-cell epitopes variability.

Our results revealed that while the general fold is conserved among profilins, substantial structural differences were found, particularly affecting the special distribution and length of different 2-D structural elements (i.e. cysteine residues), characteristic loops and coils, and numerous micro-heterogeneities present in fundamental residues directly involved in the interacting motifs, and to some extension these residues nearby to the ligand-interacting areas. Differential changes as result of polymorphism might contribute to generate functional variability among the plethora of profilin isoforms present in the olive pollen from different genetic background (olive cultivars), and between plant species, since biochemical interacting properties and binding affinities to natural ligands may be affected, particularly the interactions with different actin isoforms and phosphoinositides lipids species.

Furthermore, conspicuous variability in lineal and conformational epitopes was found between profilins belonging to the same olive cultivar, and among different cultivars as direct implication of sequences polymorphism. The variability of the residues taking part of IgE-binding epitopes might be the final responsible of the differences in cross-reactivity among olive pollen cultivars, among pollen and plant-derived food allergens, as well as between distantly related pollen species, leading to a variable range of allergy reactions among atopic patients. Identification and analysis of commonly shared and specific epitopes in profilin isoforms is essential to gain knowledge about the interacting surface of these epitopes, and for a better understanding of immune responses, helping design and development of rational and effective immunotherapy strategies for the treatment of allergy diseases.

Emerging topics in the cell biology of mitogen-activated protein kinases

Signaling through mitogen-activated protein kinase (MAPK) cascades is organized in complex interconnected subcellular networks. Upon MAPK activation, signals are transferred to targets in different subcellular compartments able to regulate various cellular processes. Therefore, subcellular dissection of individual MAPK modules is vital to understand how a single MAPK can simultaneously mediate many tasks and how a single stimulus can direct different MAPK modules to separated tasks. In this opinion article, we present a subcellular localization prediction of all members of Arabidopsis thaliana MAPK modules validated wherever possible with experimental data. Furthermore, we propose, that at least in part, the complexity of plant MAPK signaling can be explained by unique strategies of subcellular targeting, which will be worth investigating in the near future.

The human antimicrobial peptide LL-37, but not the mouse ortholog, mCRAMP, can stimulate signaling by poly(I:C) through a FPRL1-dependent pathway

LL-37 is an antimicrobial peptide produced by human cells that can down-regulate the lipopolysaccharide-induced innate immune responses and up-regulate double-stranded (ds) RNA-induced innate responses through Toll-like receptor 3 (TLR3). The murine LL-37 ortholog, mCRAMP, also inhibited lipopolysaccharide-induced responses, but unlike LL-37, it inhibited viral-induced responses in mouse cells. A fluorescence polarization assay showed that LL-37 was able to bind dsRNA better than mCRAMP. In the human lung epithelial cell line BEAS-2B, LL-37, but not mCRAMP, colocalized with TLR3, and the colocalization was increased in the presence of dsRNA. The presence of poly(I:C) increased the accumulation of LL-37 in Rab5 endosomes. Signaling by cells induced with both LL-37 and poly(I:C) was sensitive to inhibitors that affect clathrin-independent trafficking, whereas signaling by poly(I:C) alone was not, suggesting that the LL-37-poly(I:C) complex trafficked to signaling endosomes by a different mechanism than poly(I:C) alone. siRNA knockdown of known LL-37 receptors identified that FPRL1 was responsible for TLR3 signaling induced by LL-37-poly(I:C). These results show that LL-37 and mCRAMP have different activities in TLR3 signaling and that LL-37 can redirect trafficking of poly(I:C) to effect signaling by TLR3 in early endosomes in a mechanism that involves FPRL1.

Characterization of profilin polymorphism in pollen with a focus on multifunctionality

Profilin, a multigene family involved in actin dynamics, is a multiple partners-interacting protein, as regard of the presence of at least of three binding domains encompassing actin, phosphoinositide lipids, and poly-L-proline interacting patches. In addition, pollen profilins are important allergens in several species like Olea europaea L. (Ole e 2), Betula pendula (Bet v 2), Phleum pratense (Phl p 12), Zea mays (Zea m 12) and Corylus avellana (Cor a 2). In spite of the biological and clinical importance of these molecules, variability in pollen profilin sequences has been poorly pointed out up until now. In this work, a relatively high number of pollen profilin sequences have been cloned, with the aim of carrying out an extensive characterization of their polymorphism among 24 olive cultivars and the above mentioned plant species. Our results indicate a high level of variability in the sequences analyzed. Quantitative intra-specific/varietal polymorphism was higher in comparison to inter-specific/cultivars comparisons. Multi-optional posttranslational modifications, e.g. phosphorylation sites, physicochemical properties, and partners-interacting functional residues have been shown to be affected by profilin polymorphism. As a result of this variability, profilins yielded a clear taxonomic separation between the five plant species. Profilin family multifunctionality might be inferred by natural variation through profilin isovariants generated among olive germplasm, as a result of polymorphism. The high variability might result in both differential profilin properties and differences in the regulation of the interaction with natural partners, affecting the mechanisms underlying the transmission of signals throughout signaling pathways in response to different stress environments. Moreover, elucidating the effect of profilin polymorphism in adaptive responses like actin dynamics, and cellular behavior, represents an exciting research goal for the future.

Microtubules and mitogen-activated protein kinase signalling

Subcellular signalling by mitogen-activated protein kinases (MAPKs) was originally regarded as a means to regulate microtubule (MT) organization and dynamics, but with time MAPKs were assigned to broader roles concerning biotic and abiotic signal transductions. MAPKs, which regulate a broad spectrum of substrates including transcription factors and cytoskeletal proteins, belong to complex MAPK cascades, which are mainly involved in plant development and in plant stress responses. The fact that single MAPK can be regulated by more than a single MAPKKK/MAPKK pair make MAPK signalling modules versatile tools in the regulation of microtubule organization. Until recently, the best-studied MAPK module implicated in cytoskeletal regulation is the NACK-PQR pathway in tobacco (Nicotiana tabacum). Homologues of each constituent of this pathway were also discovered in Arabidopsis thaliana. So far, direct phosphorylation of tubulins by MAPKs has not been shown. However, the first MAPK-related substrate involved in the regulation of MT dynamics to have been identified is MT-associated protein MAP65-1.

Stress-responsive mitogen-activated protein kinases interact with the EAR motif of a poplar zinc finger protein and mediate its degradation through the 26S proteasome

Mitogen-activated protein kinases (MAPKs) contribute to the establishment of plant disease resistance by regulating downstream signaling components, including transcription factors. In this study, we identified MAPK-interacting proteins, and among the newly discovered candidates was a Cys-2/His-2-type zinc finger protein named PtiZFP1. This putative transcription factor belongs to a family of transcriptional repressors that rely on an ERF-associated amphiphilic repression (EAR) motif for their repression activity. Amino acids located within this repression motif were also found to be essential for MAPK binding. Close examination of the primary protein sequence revealed a functional bipartite MAPK docking site that partially overlaps with the EAR motif. Transient expression assays in Arabidopsis (Arabidopsis thaliana) protoplasts suggest that MAPKs promote PtiZFP1 degradation through the 26S proteasome. Since features of the MAPK docking site are conserved among other EAR repressors, our study suggests a novel mode of defense mechanism regulation involving stress-responsive MAPKs and EAR repressors.

The chloroplast outer membrane protein CHUP1 interacts with actin and profilin

Chloroplasts accumulate in response to low light, whereas high light induces an actin-dependent avoidance movement. This is a long known process, but its molecular base is barely understood. Only recently first components of the blue light perceiving signal cascade initiating this process were described. Among these, a protein was identified by the analysis of a deletion mutant in the corresponding gene resulting in a chloroplast unusual positioning phenotype. The protein was termed CHUP1 and initial results suggested chloroplast localization. We demonstrate that the protein is indeed exclusively and directly targeted to the chloroplast surface. The analysis of the deletion mutant of CHUP1 using microarray analysis shows an influence on the expression of genes found to be up-regulated, but not on genes found to be down-regulated upon high light exposure in wild-type. Analyzing a putative role of CHUP1 as a linker between chloroplasts and the cytoskeleton, we demonstrate an interaction with actin, which is independent on the filamentation status of actin. Moreover, binding of CHUP1 to profilin -- an actin modifying protein -- could be shown and an enhancing effect of CHUP1 on the interaction of profilin to actin is demonstrated. Therefore, a role of CHUP1 in bridging chloroplasts to actin filaments and a regulatory function in actin polymerization can be discussed.

The function of actin-binding proteins in pollen tube growth

Pollen tube growth is a key step in sexual reproduction of higher plants. The pollen tube is a typical example of tip-growing cells and shows a polarized cytoplasm. To develop and maintain polarized growth, pollen tubes need a carefully regulated actin cytoskeleton. It is well known that actin-binding proteins are responsible for the direct control of dynamic actin filaments and serve as a link between signal transduction pathways and dynamic actin changes in determining cellular architecture. Several of these classes have been identified in pollen tubes and their detailed characterisation is progressing rapidly. Here, we aim to survey what is known about the major actin-binding proteins that affect actin assembly and dynamics, and their higher-order organisation in pollen tube growth.

Profilin tyrosine phosphorylation in poly-L-proline-binding regions inhibits binding to phosphoinositide 3-kinase in Phaseolus vulgaris

The profilin family consists of a group of ubiquitous highly conserved 12-15 kDa eukaryotic proteins that bind actin, phosphoinositides, poly-l-proline (PLP) and proteins with proline-rich motifs. Some proteins with proline-rich motifs form complexes that have been implicated in the dynamics of the actin cytoskeleton and processes such as vesicular trafficking. A major unanswered question in the field is how profilin achieves the required specificity to bind such an array of proteins. It is now becoming clear that profilin isoforms are subject to differential regulation and that they may play distinct roles within the cell. Considerable evidence suggests that these isoforms have different functional roles in the sorting of diverse proteins with proline-rich motifs. All profilins contain highly conserved aromatic residues involved in PLP binding which are presumably implicated in the interaction with proline-rich motif proteins. We have previously shown that profilin is phosphorylated on tyrosine residues. Here, we show that profilin can bind directly to Phaseolus vulgaris phosphoinositide 3-kinase (PI3K) type III. We demonstrate that a new region around Y72 of profilin, as well as the N- and C-terminal PLP-binding domain, recognizes and binds PLP and PI3K. In vitro binding assays indicate that PI3K type III forms a complex with profilin in a manner that depends on the tyrosine phosphorylation status within the proline-rich-binding domain in profilin. Profilin-PI3K type III interaction suggests that profilin may be involved in membrane trafficking and in linking the endocytic pathway with actin reorganization dynamics.

Emerging MAP kinase pathways in plant stress signalling

Mitogen-activated protein kinase (MAPK) pathways transfer information from sensors to cellular responses in all eukaryotes. A surprisingly large number of genes encoding MAPK pathway components have been uncovered by analysing model plant genomes, suggesting that MAPK cascades are abundant players of signal transduction. Recent investigations have confirmed major roles of defined MAPK pathways in development, cell proliferation and hormone physiology, as well as in biotic and abiotic stress signalling. Latest insights and findings are discussed in the context of novel MAPK pathways in plant stress signalling.