HEAT SHOCK PROTEIN 90C is a bona fide Hsp90 that interacts with plastidic HSP70B in Chlamydomonas reinhardtii

Heat shock factor 1 is a key regulator of the stress response in Chlamydomonas

We report here on the characterization of heat shock factor 1 (HSF1), encoded by one of two HSF genes identified in the genome of Chlamydomonas reinhardtii. Chlamydomonas HSF1 shares features characteristic of class A HSFs of higher plants. HSF1 is weakly expressed under non-stress conditions and rapidly induced by heat shock. Heat shock also resulted in hyperphosphorylation of HSF1, and the extent of phosphorylation correlated with the degree of induction of heat shock genes, suggesting a role for phosphorylation in HSF1 activation. HSF1, like HSFs in yeasts, forms high-molecular-weight complexes, presumably trimers, under non-stress, stress and recovery conditions. Immunoprecipitation of HSF1 under these conditions led to the identification of cytosolic HSP70A as a protein constitutively interacting with HSF1. Strains in which HSF1 was strongly under-expressed by RNAi were highly sensitive to heat stress. 14C-labelling of nuclear-encoded proteins under heat stress revealed that synthesis of members of the HSP100, HSP90, HSP70, HSP60 and small HSP families in the HSF1-RNAi strains was dramatically reduced or completely abolished. This correlated with a complete loss of HSP gene induction at the RNA level. These data suggest that HSF1 is a key regulator of the stress response in Chlamydomonas.

The NH2-terminal Domain of the Chloroplast GrpE Homolog CGE1 Is Required for Dimerization and Cochaperone Function in Vivo

GrpE proteins function as nucleotide exchange factors for DnaK-type Hsp70s. We have previously identified a chloroplast homolog of GrpE in Chlamydomonas reinhardtii, termed CGE1. CGE1 exists as two isoforms, CGE1a and CGE1b, which are generated by temperature-dependent alternative splicing. CGE1b contains additional valine and glutamine residues in its extreme NH2-terminal region. Here we show that CGE1a is predominant at lower temperatures but that CGE1b becomes as abundant as CGE1a at elevated temperatures. Coimmunoprecipitation experiments revealed that CGE1b had a approximately 25% higher affinity for its chloroplast chaperone partner HSP70B than CGE1a. Modeling of the structure of CGE1b revealed that the extended alpha-helix formed by GrpE NH2 termini is 34 amino acids longer in CGE1 than in Escherichia coli GrpE and appears to contain a coiled coil motif. Progressive deletions of this coiled coil increasingly impaired the ability of CGE1 to form dimers, to interact with DnaK at elevated temperatures, and to complement temperature-sensitive growth of a DeltagrpE E. coli strain. In contrast, deletion of the four-helix bundle required for dimerization of E. coli GrpE did not affect CGE1 dimer formation. Circular dichroism measurements revealed that CGE1, like GrpE, undergoes two thermal transitions, the first of which is in the physiologically relevant temperature range (midpoint approximately 45 degrees C). Truncating the NH2-terminal coiled coil shifted the second transition to lower temperatures, whereas removal of the four-helix bundle abolished the first transition. Our data suggest that bacterial GrpE and chloroplast CGE1 share similar structural and biochemical properties, but some of these, like dimerization, are realized by different domains.

The chloroplast HSP70B-CDJ2-CGE1 chaperones catalyse assembly and disassembly of VIPP1 oligomers in Chlamydomonas

The vesicle-inducing protein in plastids (VIPP1) is essential for the biogenesis of thylakoid membranes in cyanobacteria and plants. VIPP1 and its bacterial ancestor PspA form large homo-oligomeric rings of >1 MDa. We recently demonstrated that VIPP1 interacts with the chloroplast J-domain co-chaperone CDJ2 and its chaperone partner HSP70B, and hypothesized that the chaperones might be involved in the assembly and/or disassembly of VIPP1 oligomers. To test this hypothesis, we analysed the composition of VIPP1/chaperone complexes in Chlamydomonas reinhardtii cell extracts and monitored effects of the chaperones on VIPP1 assembly states in vitro. We found that CGE1, the chloroplast GrpE homologue, is also part of complexes with HSP70B, CDJ2 and VIPP1. We observed that CDJ2-VIPP1 accumulated as low- and high-molecular-weight complexes in ATP-depleted cell extracts, but as intermediate-size complexes in extracts supplemented with ATP. This was consistent with a role for the chaperones in VIPP1 assembly and disassembly. Using purified proteins, we could demonstrate that the chaperones indeed facilitated both the assembly and disassembly of VIPP1 oligomers. Electron microscopy studies revealed that, in contrast to PspA, VIPP1 rings assembled into rod-shaped supercomplexes that were morphologically similar to microtubule-like structures observed earlier in various plastid types. VIPP1 rods, too, were disassembled by the chaperones, and chaperone-mediated rod disassembly also occurred when VIPP1 lacked an approximately 30-aa C-terminal extension present in VIPP1 homologues but absent in PspA. By regulating the assembly state of VIPP1, the chloroplast HSP70 chaperone system may play an important role in the maintenance/biogenesis of thylakoid membranes.

Studies on the role of HtpG in the tetrapyrrole biosynthesis pathway of the cyanobacterium Synechococcus elongatus PCC 7942

In cyanobacterium Synechococcus elongatus PCC 7942, we observed that htpG-overexpression caused remarkable growth inhibition. In addition, subcellular fractionation experiments showed that HtpG was localized in the membrane fraction. To understand its function in cyanobacteria, we carried out yeast two-hybrid screening to identify specific proteins interacting with HtpG, and found out, HemE, uroporphyrinogen decarboxylase. When compared to the wild-type strain, the htpG-null mutant and -overexpressing strains exhibited higher and lower cytosolic HemE activity, based on the coproporphyrin production, respectively. These results strongly suggest that HtpG is involved in the regulation of tetrapyrrole biosynthesis through interacting with HemE protein.

Advances in plant proteomics

With the avalanche of genomic information and improvements in analytical technology, proteomics is becoming increasingly important for the study of many different aspects of plant functions. Since proteins serve as important components of major signaling and biochemical pathways, studies at protein levels are essential to reveal molecular mechanisms underlying plant growth, development, and interactions with the environment. The plant proteome is highly complex and dynamic. Although great strides need to be taken towards the ultimate goal of characterizing all the proteins in a proteome, current technologies have provided immense opportunities for high-throughput proteomic studies that have gone beyond simple protein identification to analyzing various functional aspects, such as quantification, PTM, subcellular localization, and protein-protein interactions. In this review of plant proteomics, advances in protein fractionation, separation, and MS will be outlined. Focus will be on recent development in functional analysis of plant proteins, which paves the way towards the comprehensive integration with transcriptomics, metabolomics, and other large scale "-omics" into systems biology.

Comparative genomics and evolution of the HSP90 family of genes across all kingdoms of organisms

BACKGROUND

HSP90 proteins are essential molecular chaperones involved in signal transduction, cell cycle control, stress management, and folding, degradation, and transport of proteins. HSP90 proteins have been found in a variety of organisms suggesting that they are ancient and conserved. In this study we investigate the nuclear genomes of 32 species across all kingdoms of organisms, and all sequences available in GenBank, and address the diversity, evolution, gene structure, conservation and nomenclature of the HSP90 family of genes across all organisms.

RESULTS

Twelve new genes and a new type HSP90C2 were identified. The chromosomal location, exon splicing, and prediction of whether they are functional copies were documented, as well as the amino acid length and molecular mass of their polypeptides. The conserved regions across all protein sequences, and signature sequences in each subfamily were determined, and a standardized nomenclature system for this gene family is presented. The proeukaryote HSP90 homologue, HTPG, exists in most Bacteria species but not in Archaea, and it evolved into three lineages (Groups A, B and C) via two gene duplication events. None of the organellar-localized HSP90s were derived from endosymbionts of early eukaryotes. Mitochondrial TRAP and endoplasmic reticulum HSP90B separately originated from the ancestors of HTPG Group A in Firmicutes-like organisms very early in the formation of the eukaryotic cell. TRAP is monophyletic and present in all Animalia and some Protista species, while HSP90B is paraphyletic and present in all eukaryotes with the exception of some Fungi species, which appear to have lost it. Both HSP90C (chloroplast HSP90C1 and location-undetermined SP90C2) and cytosolic HSP90A are monophyletic, and originated from HSP90B by independent gene duplications. HSP90C exists only in Plantae, and was duplicated into HSP90C1 and HSP90C2 isoforms in higher plants. HSP90A occurs across all eukaryotes, and duplicated into HSP90AA and HSP90AB in vertebrates. Diplomonadida was identified as the most basal organism in the eukaryote lineage.

CONCLUSION

The present study presents the first comparative genomic study and evolutionary analysis of the HSP90 family of genes across all kingdoms of organisms. HSP90 family members underwent multiple duplications and also subsequent losses during their evolution. This study established an overall framework of information for the family of genes, which may facilitate and stimulate the study of this gene family across all organisms.