Differential influence of ATP on native spinach 70-kilodalton heat-shock cognates

Identification of Interactions between Proteins Encoded by Grapevine Leafroll-Associated Virus 3

The roles of proteins encoded by members of the genus Ampelovirus, family Closteroviridae are largely inferred by sequence homology or analogy to similarly located ORFs in related viruses. This study employed yeast two-hybrid and bimolecular fluorescence complementation assays to investigate interactions between proteins of grapevine leafroll-associated virus 3 (GLRaV-3). The p5 movement protein, HSP70 homolog, coat protein, and p20B of GLRaV-3 were all found to self-interact, however, the mechanism by which p5 interacts remains unknown due to the absence of a cysteine residue crucial for the dimerisation of the closterovirus homolog of this protein. Although HSP70h forms part of the virion head of closteroviruses, in GLRaV-3, it interacts with the coat protein that makes up the body of the virion. Silencing suppressor p20B has been shown to interact with HSP70h, as well as the major coat protein and the minor coat protein. The results of this study suggest that the virion assembly of a member of the genus Ampelovirus occurs in a similar but not identical manner to those of other genera in the family Closteroviridae. Identification of interactions of p20B with virus structural proteins provides an avenue for future research to explore the mechanisms behind the suppression of host silencing and suggests possible involvement in other aspects of the viral replication cycle.

Hsp70 oligomerization is mediated by an interaction between the interdomain linker and the substrate-binding domain

Oligomerization in the heat shock protein (Hsp) 70 family has been extensively documented both in vitro and in vivo, although the mechanism, the identity of the specific protein regions involved and the physiological relevance of this process are still unclear. We have studied the oligomeric properties of a series of human Hsp70 variants by means of nanoelectrospray ionization mass spectrometry, optical spectroscopy and quantitative size exclusion chromatography. Our results show that Hsp70 oligomerization takes place through a specific interaction between the interdomain linker of one molecule and the substrate-binding domain of a different molecule, generating dimers and higher-order oligomers. We have found that substrate binding shifts the oligomerization equilibrium towards the accumulation of functional monomeric protein, probably by sequestering the helical lid sub-domain needed to stabilize the chaperone: substrate complex. Taken together, these findings suggest a possible role of chaperone oligomerization as a mechanism for regulating the availability of the active monomeric form of the chaperone and for the control of substrate binding and release.

Inhibition of protein synthesis by TOR inactivation revealed a conserved regulatory mechanism of the BiP chaperone in Chlamydomonas

The target of rapamycin (TOR) kinase integrates nutritional and stress signals to coordinately control cell growth in all eukaryotes. TOR associates with highly conserved proteins to constitute two distinct signaling complexes termed TORC1 and TORC2. Inactivation of TORC1 by rapamycin negatively regulates protein synthesis in most eukaryotes. Here, we report that down-regulation of TOR signaling by rapamycin in the model green alga Chlamydomonas reinhardtii resulted in pronounced phosphorylation of the endoplasmic reticulum chaperone BiP. Our results indicated that Chlamydomonas TOR regulates BiP phosphorylation through the control of protein synthesis, since rapamycin and cycloheximide have similar effects on BiP modification and protein synthesis inhibition. Modification of BiP by phosphorylation was suppressed under conditions that require the chaperone activity of BiP, such as heat shock stress or tunicamycin treatment, which inhibits N-linked glycosylation of nascent proteins in the endoplasmic reticulum. A phosphopeptide localized in the substrate-binding domain of BiP was identified in Chlamydomonas cells treated with rapamycin. This peptide contains a highly conserved threonine residue that might regulate BiP function, as demonstrated by yeast functional assays. Thus, our study has revealed a regulatory mechanism of BiP in Chlamydomonas by phosphorylation/dephosphorylation events and assigns a role to the TOR pathway in the control of BiP modification.

Crystal structures of the 70-kDa heat shock proteins in domain disjoining conformation

The 70-kDa heat shock proteins (Hsp70s) are highly conserved ATP-dependent molecular chaperones composed of an N-terminal nucleotide binding domain (NBD) and a C-terminal protein substrate binding domain (SBD) in a bilobate structure. Interdomain communication and nucleotide-dependent structural motions are critical for Hsp70 chaperone functions. Our understanding of these functions remains elusive due to insufficient structural information on intact Hsp70s that represent the different states of the chaperone cycle. We report here the crystal structures of DnaK from Geobacillus kaustophilus HTA426 bound with ADP-Mg(2+)-P(i) at 2.37A and the 70-kDa heat shock cognate protein from Rattus norvegicus bound with ADP-P(i) at 3.5A(.) The NBD and SBD in these structures are significantly separated from each other, and they might depict the ADP-bound conformation. Moreover, a Trp reporter was introduced at the potential interface region between NBD and the interdomain linker of GkDnaK to probe environmental changes. Results from fluorescence measurements support the notion that substrate binding enhances the domain-disjoining behavior of Hsp70 chaperones.

Purification and characterisation of a jacalin-related, coleoptile specific lectin from Hordeum vulgare

A plant lectin was isolated from barley (Hordeum vulgare) coleoptiles using acidic extraction and different chromatographic methods. Sequencing of more than 50% of the protein sequence by Edman degradation confirmed a full-length cDNA clone. The subsequently identified open reading frame encodes for a 15 kDa protein which could be found in the soluble fraction of barley coleoptiles. This protein exhibited specificity towards mannose sugar and is therefore, accordingly named as Horcolin (Hordeum vulgare coleoptile lectin). Database searches performed with the Horcolin protein sequence revealed a sequence and structure homology to the lectin family of jacalin-related lectins. Together with its affinity towards mannose, Horcolin is now identified as a new member of the mannose specific subgroup of jacalin-related lectins in monocot species. Horcolin shares a high amino acid homology to the highly light-inducible protein HL#2 and, in addition to two methyl jasmonic acid-inducible proteins of 32.6 and 32.7 kDa where the jasmonic acid-inducible proteins are examples of bitopic chimerolectins containing a dirigent and jacalin-related domain. Immunoblot analysis with a cross-reactive anti-HL#2 antibody in combination with Northern blot analysis of the Horcolin cDNA revealed tissue specific expression of Horcolin in the coleoptiles. The function of Horcolin is discussed in the context of its particular expression in coleoptiles and is then compared to other lectins, which apparently share a related response to biotic or abiotic stress factors.

In vitro evidence of Hsc70 functioning as a molecular chaperone during cold stress

Hsp70 molecular chaperones have been shown to play an important role in helping cells to cope with adverse environments, especially in response to high temperatures. The molecular chaperone function of Hsc70 at low temperature was investigated. A cold-inducible spinach cytosolic Hsc70 was subcloned into a protein expression vector and the recombinant protein was expressed in bacterial cells. Recombinant Hsc70 bound a permanently unfolded substrate: alpha-carboxymethylated lactalbumin (CMLA) in the presence of 3 mM ATP and MgCl(2) at low temperature (4 and -4 degrees C). Radiolabeling with (35)S-Met and (35)S-Cys and immunoprecipitation with cytosolic Hsc70 monoclonal antibodies showed that there were several proteins co-immunoprecipitated at low temperature (4 and -4 degrees C) but not at room temperature. Enhanced co-purification of sHsp17.7 with Hsc70 at low temperature was observed and suggests that co-chaperone interactions can contribute to molecular chaperone function during cold stress. These results suggest that the molecular chaperone Hsc70 may have a functional role in plants during low temperature stress.

Co-immunoprecipitation of Hsp101 with cytosolic Hsc70

In animals and yeast, cytosolic Hsp70s function in concert with other molecular chaperones. Hsp70 is a major chaperone in the Hsp90 multi-chaperone complexes that participate in maturation of steroid receptors and several other proteins. Hsp70s also appear to form a complex with Hsp90 and Hsp110/sHsp. A 100 kDa protein was co-immunoprecipitated with cytosolic Hsc70 from maize seedlings (Zea mays). The presence of this complex was further confirmed using gel-filtration chromatography. Mass spectrometric analysis showed that the 100 kDa protein is homologous with Arabidopsis Hsp101. Treatment with apyrase enhanced the co-immunoprecipitation of Hsp101 with Hsc70, while ATP had the opposite effect. In the presence of carboxymethylated alpha-lactalbumin (CMLA), which is permanently unfolded, the complex dissociated. Based on these observations, it is concluded that Hsc70 and Hsp101 are present in a complex in the plant cytosol.

Autophosphorylation and subcellular localization dynamics of a salt- and water deficit-induced calcium-dependent protein kinase from ice plant

A salinity and dehydration stress-responsive calcium-dependent protein kinase (CDPK) was isolated from the common ice plant (Mesembryanthemum crystallinum; McCPK1). McCPK1 undergoes myristoylation, but not palmitoylation in vitro. Removal of the N-terminal myristate acceptor site partially reduced McCPK1 plasma membrane (PM) localization as determined by transient expression of green fluorescent protein fusions in microprojectile-bombarded cells. Removal of the N-terminal domain (amino acids 1-70) completely abolished PM localization, suggesting that myristoylation and possibly the N-terminal domain contribute to membrane association of the kinase. The recombinant, Escherichia coli-expressed, full-length McCPK1 protein was catalytically active in a calcium-dependent manner (K0.5 = 0.15 microm). Autophosphorylation of recombinant McCPK1 was observed in vitro on at least two different Ser residues, with the location of two sites being mapped to Ser-62 and Ser-420. An Ala substitution at the Ser-62 or Ser-420 autophosphorylation site resulted in a slight increase in kinase activity relative to wild-type McCPK1 against a histone H1 substrate. In contrast, Ala substitutions at both sites resulted in a dramatic decrease in kinase activity relative to wild-type McCPK1 using histone H1 as substrate. McCPK1 undergoes a reversible change in subcellular localization from the PM to the nucleus, endoplasmic reticulum, and actin microfilaments of the cytoskeleton in response to reductions in humidity, as determined by transient expression of McCPK1-green fluorescent protein fusions in microprojectile-bombarded cells and confirmed by subcellular fractionation and western-blot analysis of 6x His-tagged McCPK1.

The COOH-terminal peptide binding domain is essential for self-association of the molecular chaperone HSC70

We have previously shown that the molecular chaperone HSC70 self-associates in solution into dimers, trimers, and probably high order oligomers, according to a slow temperature- and concentration-dependent equilibrium that is shifted toward the monomer upon binding of ATP peptides or unfolded proteins. To determine the structural basis of HSC70 self-association, the oligomerization properties of the isolated amino- and carboxyl-terminal domains of this protein have been analyzed by gel electrophoresis, size exclusion chromatography, and analytical ultracentrifugation. Whereas the amino-terminal ATPase domain (residues 1-384) was found to be monomeric in solution even at high concentrations, the carboxyl-terminal peptide binding domain (residues 385-646) exists as a slow temperature- and concentration-dependent equilibrium involving monomers, dimers, and trimers. The association equilibrium constant obtained for this domain alone is on the order of 10(5) M-1, very close to that determined previously for the entire protein, suggesting that self-association of HSC70 is determined solely by its carboxyl-terminal domain. Furthermore, oligomerization of the isolated carboxyl-terminal peptide binding domain is, like that of the entire protein, reversed by peptide binding, indicating that self-association of the protein may be mediated by the peptide binding site and, as such, should play a role in the regulation of HSC70 chaperone function. A general model for self-association of HSP70 is proposed in which the protein is in equilibrium between two states differing by the conformation of their carboxyl-terminal domain and their self-association properties.

Molecular chaperones and protein folding in plants

Protein folding in vivo is mediated by an array of proteins that act either as 'foldases' or 'molecular chaperones'. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.

Molecular chaperones and protein folding in plants

Protein folding in vivo is mediated by an array of proteins that act either as 'foldases' or 'molecular chaperones'. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.

Effect of nucleotides, peptides, and unfolded proteins on the self-association of the molecular chaperone HSC70

In a previous study, we showed that the molecular chaperone HSC70 self-associates in solution in a reversible and likely unlimited fashion. Here, we examine the influence of nucleotides, nucleotide analogs, peptides, and unfolded proteins on the self-association properties of this protein. Whereas in the presence of ADP, HSC70 exists as a slow, concentration- and temperature-dependent monomer-oligomer equilibrium, in the presence of ATP, the protein is essentially monomeric, indicating that ATP shifts this equilibrium toward the monomer by stabilizing the monomer. Dissociation of oligomers into monomers is also obtained with the slowly hydrolyzable ATP analogs, adenosine 5'-O-(thiotriphosphate) and 5'-adenylyl-beta,gamma-imidodiphosphate, or the complex between ADP and the phosphate analog, BeF3, indicating that binding but not hydrolysis of ATP is necessary and sufficient for the stabilization of HSC70 monomer. Furthermore, binding of short peptides or permanently unfolded proteins to the peptide binding site of HSC70 promotes the dissociation of oligomers into monomers, suggesting that protein substrates are able to compete with HSC70 for the same binding site. Because the release of peptides or unfolded proteins from HSC70 has also been shown to require ATP binding, these results indicate that dissociation of oligomers is controlled by a mechanism similar to that of release of protein substrates and suggest that binding of HSC70 to itself occurs via the peptide binding site and mimics binding of HSC70 to protein substrates.

COMPARTMENTATION OF PROTEINS IN THE ENDOMEMBRANE SYSTEM OF PLANT CELLS

This review focuses on four interrelated processes in the plant endomembrane system: compartmentation of proteins in subdomains of the endoplasmic reticulum, mechanisms that determine whether storage proteins are retained within the ER lumen or transported out, the origin and function of biochemically distinct vacuoles or prevacuolar organelles, and the cellular processes by which proteins are sorted to vacuolar compartments. We postulate that ER-localized protein bodies are formed by a series of orderly events of protein synthesis, protein concentration, and protein assembly in subdomains of the ER. Protein concentration, which facilitates protein-to-protein interactions and subsequent protein assembly, may be achieved by the interactions with chaperones and by the localization of storage protein mRNAs. We also describe recent developments on the coexistence of two biochemically distinguishable vacuolar compartments, the possible direct role of the ER in vacuole biogenesis, and proposed mechanisms for transport of proteins from the ER or Golgi apparatus to the vacuole.

ATPase activity and molecular chaperone function of the stress70 proteins

The codons for the amino acid residues making up the proposed ATP-binding sites of the maize (Zea mays L.) endoplasmic reticulum and tomato (Lycopersicon esculentum) cytoplasmic Stress70 proteins were deleted from their respective cDNAs. The deletions had little effect on the predicted secondary structure characteristics of the encoded proteins. Both wild-type and mutant proteins were expressed in Escherichia coli and purified to electrophoretic homogeneity. The mutant recombinant proteins did not bind to immobilized ATP columns, had no detectable ATPase activity, and were unable to function in vitro as molecular chaperones. Additionally, the inability to bind ATP was associated with changes in the oligomerization state of the Stress70 proteins.

Spinach leaf 70-kilodalton heat-shock cognate stabilizes bovine adrenal glucose-6-phosphate dehydrogenase in vitro without apparent stable binding

Spinach (Spinacia oleracea L.) leaf tissue 70-kilodalton heat-shock cognate was purified by ATP-agarose affinity and gel filtration. Gel filtration of the affinity-purified protein resolved it into three forms: monomer, dimer, and oligomer. In the absence of ATP, the majority of the heat-shock cognate existed as a monomeric form with lesser amounts of dimer and oligomer. Addition of 3 mM ATP to the purified protein, containing all three forms, converted the dimeric and monomeric forms to a high-molecular-weight complex. Removal of ATP from the complex by dialysis resulted in the reappearance of the dimeric and monomeric forms. Addition of ATP to the highly purified monomer had no effect on its gel-filtration migration. Neither purified monomeric or dimeric forms showed stable binding to denatured proteins; however, both forms of the purified heat-shock cognate were able to stabilize the enzymatic activity of bovine adrenal glucose-6-phosphate dehydrogenase over a 48-h period at 25 degrees C. In addition, the activity of glucose-6-phosphate dehydrogenase in the presence of purified heat-shock cognate dimer or monomer could be rapidly decreased in an ATP-dependent fashion depending on the order of the substrate addition to the reaction mixture. Circular-dichroism studies indicated that addition of ATP to the spinach 70-kDa heat-shock cognate caused a conformation change from alpha-helical to a greater beta-sheet content. How conformational character may influence the stabilizing activity of the heat-shock cognate in a mechanism which does not require stable peptide binding is discussed.

Structural organization of the spinach endoplasmic reticulum-luminal 70-kilodalton heat-shock cognate gene and expression of 70-kilodalton heat-shock genes during cold acclimation

The 70-kD heat-shock proteins (HSP70s) are encoded by a multigene family in eukaryotes. In plants, the 70-kD heat-shock cognate (HSC70) proteins are located in organellar and cytosolic compartments of cells in most tissues. Previous work has indicated that HSC70 proteins of spinach (Spinacia oleracea) are actively synthesized during cold-acclimating conditions. We have isolated, sequenced, and characterized cDNA and genomic clones for the endoplasmic reticulum (ER) luminal HSC70 protein (immunoglobulin heavy chain-binding protein; BiP) of spinach. The spinach ER-luminal HSC70 is a constitutively expressed gene consisting of eight exons. Spinach BiP mRNA appears to be up-regulated during cold acclimation but is not expressed during water stress or heat shock. In contrast to the differential regulation of mRNA, the ER-luminal HSC70 protein levels remain constant in response to various environmental stresses. Two other members of the spinach 70-kD heat-shock (HS70) multigene family also show differential expression in response to a variety of environmental stresses. A constitutively expressed cytosolic HSC70 protein in spinach appears also to be up-regulated in response to both cold-acclimating and heat-shock treatments. Spinach also contains a cold-shock-induced HS70 gene that is not expressed during heat shock or water stress. Since HSP70s are considered to be involved with the chaperoning and folding of proteins, the data further support the concept that they may be important for maintaining cellular homeostasis and proper protein biogenesis during cold acclimation of spinach.