Class I and II Small Heat Shock Proteins Together with HSP101 Protect Protein Translation Factors during Heat Stress

Comparative transcriptome analysis reveals phytohormone signalings, heat shock module and ROS scavenger mediate the cold-tolerance of rubber tree

Two contrasting cold response rubber tree clones, the cold-resistant ‘93-114’ and cold-sensitive ‘Reken501’, were subject to a global transcriptome response assessing via high-throughput RNA-seq technique and comprehensive bioinformatics analysis using the referenced rubber tree genome with the purpose of exploring the potential molecular cues underlying the tolerance of rubber trees to cold stress. As a result, a total of 1919 genes had significantly higher expression, while 2929 genes had significantly lower expression in ‘93–114’ than in ‘Reken501’ without cold stress. Upon cold stress, the numbers of genes with significantly higher expression decreased to 1501 at 1 h treatment and to 1285 at 24 h treatment in ‘93–114’ than that of ‘Reken501’, conversely, the numbers of genes with significantly lower expression increased to 7567 at 1 h treatment and to 5482 at 24 h treatment. Functional annotation of the differentially expressed genes between ‘93–114’ and ‘Reken501’ suggests that down-regulation of auxin and ethylene signaling and activation of heat shock module and ROS scavengers is a primary strategy for H. brasiliensis to cope with cold stress. Our identified vital differentially expressed genes may be beneficial for elucidation of the molecular mechanisms underlying cold tolerance and for genetic improvement of H. brasiliensis clones.

Structural principles that enable oligomeric small heat-shock protein paralogs to evolve distinct functions

Oligomeric proteins assemble with exceptional selectivity, even in the presence of closely related proteins, to perform their cellular roles. We show that most proteins related by gene duplication of an oligomeric ancestor have evolved to avoid hetero-oligomerization and that this correlates with their acquisition of distinct functions. We report how coassembly is avoided by two oligomeric small heat-shock protein paralogs. A hierarchy of assembly, involving intermediates that are populated only fleetingly at equilibrium, ensures selective oligomerization. Conformational flexibility at noninterfacial regions in the monomers prevents coassembly, allowing interfaces to remain largely conserved. Homomeric oligomers must overcome the entropic benefit of coassembly and, accordingly, homomeric paralogs comprise fewer subunits than homomers that have no paralogs.

Heat in Wheat: Exploit Reverse Genetic Techniques to Discover New Alleles Within the Triticum durum sHsp26 Family

Wheat breeding nowadays must address producers and consumers' desire. According to the last FAO report, a dramatic decrease in wheat production is expected in the next decades mainly due to the upcoming climate change. The identification of the processes which are triggered by heat stress and how thermotolerance develops in wheat is an active research topic. Genomic approach may help wheat breeding since it allows direct study on the genotype and relationship with the phenotype. Here the isolation and characterization of four members of the chloroplast-localized small heat shock proteins (sHSP) encoded by the Hsp26 gene family is reported. Furthermore, two high throughput TILLING (Targeting Induced Local Lesions In Genomes) approaches in vivo and in silico were used for the identification of new alleles within this family. Small heat shock proteins are known to prevent the irreversible aggregation of misfolded proteins and contribute to the acquisition of thermotolerance. Chloroplast-localized sHSPs protect the photosynthetic machinery during episodes of high temperature stress. The modulation of the newly discovered genes within the sHsp26 family has been analyzed in vivo and by the ExpVIP platform widening the abiotic stress analysis; and their involvement in the heat stress response has been demonstrated. In addition, in this study a total of 50 TILLING mutant lines have been identified. A set of KASP (Kompetitive Allele Specific PCR) markers was also developed to follow the specific mutations in the ongoing backcrosses, applicable to high throughput genotyping approaches and usable in marker assisted selection breeding programs.

Role of sHsps in organizing cytosolic protein aggregation and disaggregation

Small heat shock proteins (sHsps) exhibit an ATP-independent chaperone activity to prevent the aggregation of misfolded proteins in vitro. The seemingly conflicting presence of sHsps in insoluble protein aggregates in cells obstructs a precise definition of sHsp function in proteostasis networks. Recent findings specify sHsp activities in protein quality control systems. The sHsps of yeast, Hsp42 and Hsp26, interact with early unfolding intermediates of substrates, keeping them in a ready-to-refold conformation close to the native state. This activity facilitates substrate refolding by ATP-dependent Hsp70-Hsp100 disaggregating chaperones. Hsp42 can actively sequester misfolded proteins and promote their deposition at specific cellular sites. This aggregase activity represents a cytoprotective protein quality control strategy. The aggregase function of Hsp42 controls the formation of cytosolic aggregates (CytoQs) under diverse stress regimes and can be reconstituted in vitro, demonstrating that Hsp42 is necessary and sufficient to promote protein aggregation. Substrates sequestered at CytoQs can be dissociated by Hsp70-Hsp100 disaggregases for subsequent triage between refolding and degradation pathways or are targeted for destruction by selective autophagy termed proteophagy.

The growing world of small heat shock proteins: from structure to functions

Small heat shock proteins (sHSPs) are present in all kingdoms of life and play fundamental roles in cell biology. sHSPs are key components of the cellular protein quality control system, acting as the first line of defense against conditions that affect protein homeostasis and proteome stability, from bacteria to plants to humans. sHSPs have the ability to bind to a large subset of substrates and to maintain them in a state competent for refolding or clearance with the assistance of the HSP70 machinery. sHSPs participate in a number of biological processes, from the cell cycle, to cell differentiation, from adaptation to stressful conditions, to apoptosis, and, even, to the transformation of a cell into a malignant state. As a consequence, sHSP malfunction has been implicated in abnormal placental development and preterm deliveries, in the prognosis of several types of cancer, and in the development of neurological diseases. Moreover, mutations in the genes encoding several mammalian sHSPs result in neurological, muscular, or cardiac age-related diseases in humans. Loss of protein homeostasis due to protein aggregation is typical of many age-related neurodegenerative and neuromuscular diseases. In light of the role of sHSPs in the clearance of un/misfolded aggregation-prone substrates, pharmacological modulation of sHSP expression or function and rescue of defective sHSPs represent possible routes to alleviate or cure protein conformation diseases. Here, we report the latest news and views on sHSPs discussed by many of the world's experts in the sHSP field during a dedicated workshop organized in Italy (Bertinoro, CEUB, October 12-15, 2016).

Heat Shock Protein HSP101 Affects the Release of Ribosomal Protein mRNAs for Recovery after Heat Shock

Heat shock (HS) is known to have a profound impact on gene expression at different levels, such as inhibition of protein synthesis, in which HS blocks translation initiation and induces the sequestration of mRNAs into stress granules (SGs) or P-bodies for storage and/or decay. SGs prevent the degradation of the stored mRNAs, which can be reengaged into translation in the recovery period. However, little is known on the mRNAs stored during the stress, how these mRNAs are released from SGs afterward, and what the functional importance is of this process. In this work, we report that Arabidopsis HEAT SHOCK PROTEIN101 (HSP101) knockout mutant (hsp101) presented a defect in translation recovery and SG dissociation after HS Using RNA sequencing and RNA immunoprecipitation approaches, we show that mRNAs encoding ribosomal proteins (RPs) were preferentially stored during HS and that these mRNAs were released and translated in an HSP101-dependent manner during recovery. By ¹⁵N incorporation and polysome profile analyses, we observed that these released mRNAs contributed to the production of new ribosomes to enhance translation. We propose that, after HS, HSP101 is required for the efficient release of RP mRNAs from SGs resulting in a rapid restoration of the translation machinery by producing new RPs.

Pollen Development at High Temperature: From Acclimation to Collapse

Pollen development at high temperature depends on a fine balance between acclimation and injury.

Structural model of dodecameric heat-shock protein Hsp21: Flexible N-terminal arms interact with client proteins while C-terminal tails maintain the dodecamer and chaperone activity

Small heat-shock proteins (sHsps) prevent aggregation of thermosensitive client proteins in a first line of defense against cellular stress. The mechanisms by which they perform this function have been hard to define due to limited structural information; currently, there is only one high-resolution structure of a plant sHsp published, that of the cytosolic Hsp16.9. We took interest in Hsp21, a chloroplast-localized sHsp crucial for plant stress resistance, which has even longer N-terminal arms than Hsp16.9, with a functionally important and conserved methionine-rich motif. To provide a framework for investigating structure-function relationships of Hsp21 and understanding these sequence variations, we developed a structural model of Hsp21 based on homology modeling, cryo-EM, cross-linking mass spectrometry, NMR, and small-angle X-ray scattering. Our data suggest a dodecameric arrangement of two trimer-of-dimer discs stabilized by the C-terminal tails, possibly through tail-to-tail interactions between the discs, mediated through extended IXVXI motifs. Our model further suggests that six N-terminal arms are located on the outside of the dodecamer, accessible for interaction with client proteins, and distinct from previous undefined or inwardly facing arms. To test the importance of the IXVXI motif, we created the point mutant V181A, which, as expected, disrupts the Hsp21 dodecamer and decreases chaperone activity. Finally, our data emphasize that sHsp chaperone efficiency depends on oligomerization and that client interactions can occur both with and without oligomer dissociation. These results provide a generalizable workflow to explore sHsps, expand our understanding of sHsp structural motifs, and provide a testable Hsp21 structure model to inform future investigations.

Bioorthogonal Noncanonical Amino Acid Tagging (BONCAT) Enables Time-Resolved Analysis of Protein Synthesis in Native Plant Tissue

Proteomic plasticity undergirds stress responses in plants, and understanding such responses requires accurate measurement of the extent to which proteins levels are adjusted to counter external stimuli. Here, we adapt bioorthogonal noncanonical amino acid tagging (BONCAT) to interrogate protein synthesis in vegetative Arabidopsis (Arabidopsis thaliana) seedlings. BONCAT relies on the translational incorporation of a noncanonical amino acid probe into cellular proteins. In this study, the probe is the Met surrogate azidohomoalanine (Aha), which carries a reactive azide moiety in its amino acid side chain. The azide handle in Aha can be selectively conjugated to dyes and functionalized beads to enable visualization and enrichment of newly synthesized proteins. We show that BONCAT is sensitive enough to detect Arabidopsis proteins synthesized within a 30-min interval defined by an Aha pulse and that the method can be used to detect proteins made under conditions of light stress, osmotic shock, salt stress, heat stress, and recovery from heat stress. We further establish that BONCAT can be coupled to tandem liquid chromatography-mass spectrometry to identify and quantify proteins synthesized during heat stress and recovery from heat stress. Our results are consistent with a model in which, upon the onset of heat stress, translation is rapidly reprogrammed to enhance the synthesis of stress mitigators and is again altered during recovery. All experiments were carried out with commercially available reagents, highlighting the accessibility of the BONCAT method to researchers interested in stress responses as well as translational and posttranslational regulation in plants.

Identification of MsHsp20 Gene Family in Malus sieversii and Functional Characterization of MsHsp16.9 in Heat Tolerance

Heat shock proteins (Hsps) are common molecular chaperones present in all plants that accumulate in response to abiotic stress. Small heat shock proteins (sHsps) play important roles in alleviating diverse abiotic stresses, especially heat stress. However, very little is known about the MsHsp20 gene family in the wild apple Malus sieversii, a precious germplasm resource with excellent resistance characteristics. In this study, 12 putative M. sieversii Hsp20 genes were identified from RNA-Seq data and analyzed in terms of gene structure and phylogenetic relationships. A new Hsp20 gene, MsHsp16.9, was cloned and its function studied in response to stress. MsHsp16.9 expression was strongly induced by heat, and transgenic Arabidopsis plants overexpressing MsHsp16.9 displayed improved heat resistance, enhanced antioxidant enzyme activity, and decreased peroxide content. Overexpression of MsHsp16.9 did not alter the growth or development under normal conditions, or the hypersensitivity to exogenous ABA. Gene expression analysis indicated that MsHsp16.9 mainly modulates the expression of proteins involved in antioxidant enzyme synthesis, as well as ABA-independent stress signaling in 35S:MsHsp16.9-L11. However, MsHsp16.9 could activate ABA-dependent signaling pathways in all transgenic plants. Additionally, MsHsp16.9 may function alongside AtHsp70 to maintain protein homeostasis and protect against cell damage. Our results suggest that MsHsp16.9 is a protein chaperone that positively regulates antioxidant enzyme activity and ABA-dependent and independent signaling pathway to attenuate plant responses to severe stress. Transgenic plants exhibited luxuriant growth in high temperature environments.