Upregulation of cytosolic chaperonin CCT subunits during recovery from chemical stress that causes accumulation of unfolded proteins

Proteome analysis of nuclear matrix proteins during apoptotic chromatin condensation

The nuclear matrix (NM) is considered a proteinaceous scaffold spatially organizing the interphase nucleus, the integrity of which is affected during apoptosis. Caspase-mediated degradation of NM proteins, such as nuclear lamins, precedes apoptotic chromatin condensation (ACC). Nevertheless, other NM proteins remain unaffected, which most likely maintain a remaining nuclear structure devoid of chromatin. We, therefore, screened various types of apoptotic cells for changes of the nuclear matrix proteome during the process of apoptotic ACC. Expectedly, we observed fundamental alterations of known chromatin-associated proteins, comprising both degradation and translocation to the cytosol. Importantly, a consistent set of abundant NM proteins, some (e.g. hNMP 200) of which displaying structural features, remained unaffected during apoptosis and might therefore represent constituents of an elementary scaffold. In addition, proteins involved in DNA replication and DNA repair were found accumulated in the NM fraction before cells became irreversibly committed to ACC, a time point characterized in detail by inhibitor studies with orthovanadate. In general, protein alterations of a consistent set of NM proteins (67 of which were identified), were reproducibly detectable in Fas-induced Jurkat cells, in UV-light treated U937 cells and also in staurosporine-treated HeLa cells. Our data indicate that substantial alterations of proteins linking chromatin to an elementary nuclear protein scaffold might play an intriguing role for the process of ACC.

The eukaryote chaperonin CCT is a cold shock protein in Saccharomyces cerevisiae

The eukaryotic Hsp60 cytoplasmic chaperonin CCT (chaperonin containing the T-complex polypeptide-1) is essential for growth in budding yeast, and mutations in individual CCT subunits have been shown to affect assembly of tubulin and actin. The present research focused mainly on the expression of the CCT subunits, CCTalpha and CCTbeta, in yeast (Saccharomyces cerevisiae). Previous studies showed that, unlike most other chaperones, CCT in yeast does not undergo induction following heat shock. In this study, messenger ribonucleic acid (mRNA) and protein levels of CCT subunits following exposure to low temperatures, were examined. The Northern blot analysis indicated a 3- to 4-fold increase in mRNA levels of CCTalpha and CCTbeta genes after cold shock at 4 degrees C. Interestingly, Western blot analysis showed that cold shock induces an increase in the CCTalpha protein, which is expressed at 10 degrees C, but not at 4 degrees C. Transfer of 4 degrees C cold-shocked cells to 10 degrees C induced a 5-fold increase in the CCTalpha protein level. By means of fluorescent immunostaining and confocal microscopy, we found CCTalpha to be localized in the cortex and the cell cytoplasm of S. cerevisiae. Localization of CCTalpha was not affected at low temperatures. Co-localization of CCT and filaments of actin and tubulin was not observed by microscopy. The induction pattern of the CCTalpha protein suggests that expression of the chaperonin may be primarily important during the recovery from low temperatures and the transition to growth at higher temperatures, as found for other Hsps during the recovery phase from heat shock.

Characterisation of the gene encoding a protective antigen from Babesia microti identified it as eta subunit of chaperonin containing T-complex protein 1

Passive immunisations with a monoclonal antibody termed 1-5H showed a partial but significant inhibition of parasitaemia against Babesia microti challenge infection. By immunoscreening with 1-5H, a clone (termed p58 gene) was obtained from a cDNA expression library of B. microti and the complete nucleotide sequence was determined. A protein homology search showed significant amino acid identities to the eta subunit of the chaperonin containing T-complex protein 1 (CCT) of human (59%), mouse (58%) and Plasmodium falciparum (62%). Genomic analyses indicated that the p58 gene is present as a single copy gene and contains a total of approximately 400-bp introns in the genome of B. microti. The mAb 1-5H recognised a 58-kDa protein of B. microti and was found to cross-react with a 60-kDa protein of Babesia rodhaini. These results suggest the possibility that the p58 protein is the CCT eta subunit of B. microti and functions as a chaperonin.

Microtubule cytoskeleton perturbation induced by taxol and colchicine affects chaperonin containing TCP-1 (CCT) subunit gene expression in Tetrahymena cells

We report the existence of a CCT epsilon subunit gene that encodes subunit epsilon of the chaperonin CCT (chaperonin containing TCP-1) in Tetrahymena pyriformis. This work focuses on the study of the effects of the microtubule polymerizing agent taxol and the depolymerizing agent colchicine on microtubule dynamics and their role in the regulation of tubulin and CCT subunit genes. Under taxol treatment some TpCCT and tubulin genes are distinctly expressed until 30 min of treatment. Cytoplasmic TpCCT mRNA levels slightly decrease while tubulin transcripts are increasing. In colchicine treated cells TpCCT and tubulin transcripts decrease in the initial 30 min of treatment and then start to increase. However, both antimitotic agents induce TpCCT and tubulin gene transcription. This induction does not correlate with increased steady-state levels of TpCCT proteins and seems to be necessary to replete cytoplasmic TpCCT mRNAs. Moreover, we found that TpCCT epsilon and TpCCT alpha but not TpCCT eta are present in the insoluble fraction after a postmitochondrial fractionation that contains components of the ciliate cortex structure, basal bodies and cilia. This suggests that some TpCCT subunits may be associated with these structures. The association of TpCCT epsilon subunit is stimulated either by taxol or colchicine treatment. These observations support the idea that CCT subunits could have additional roles in vivo.

A novel acetyltransferase found in Saccharomyces cerevisiae Sigma1278b that detoxifies a proline analogue, azetidine-2-carboxylic acid

L-Azetidine-2-carboxylic acid (AZC), a toxic four-membered ring analogue of L-proline, is transported into the cells via proline transporters. It causes misfolding of the proteins into which it is incorporated competitively with L-proline and thereby inhibits the growth of the cells. We recently have discovered, on the chromosome of Saccharomyces cerevisiae Sigma1278b, a novel gene MPR1 required for the resistance of Sigma1278 background strains to toxic AZC. This gene was missing in the particular yeast strain used for the genomic sequence determination. Although the protein sequence was homologous to that of the S. cerevisiae transcriptional regulator, Mpr1p did not affect the expression of genes involved in proline uptake. However, gene expression in Escherichia coli and enzymatic analysis showed that the MPR1 gene encodes a novel AZC acetyltransferase, by which L-proline itself and other L-proline analogues are not acetylated. Mpr1p was considered to be a member of the N-acetyltransferase superfamily based on the results of an Ala-scan mutagenesis through the highly conserved region involved in binding acetyl-CoA in members of the superfamily. Our findings suggest that Mpr1p detoxifies AZC by acetylating it in the cytoplasm. This enzyme might be utilized as a selective marker in a wide variety of organisms, because the cells expressing the MPR1 gene acquire the AZC-resistant phenotype.

Cytosolic chaperonin-containing t-complex polypeptide 1 changes the content of a particular subunit species concomitant with substrate binding and folding activities during the cell cycle

The chaperonin-containing t-complex polypeptide 1 (CCT) is a cytosolic molecular chaperone composed of eight subunits that assists in the folding of actin, tubulin and other cytosolic proteins. We show here that the content of particular subunits of CCT within mammalian cells decreases concomitantly with the reduction of chaperone activity during cell cycle arrest at M phase. CCT recovers chaperone activity upon resumption of these subunits after release from M phase arrest or during arrest at S phase. The levels of alpha, delta and zeta-1 subunits decreased more rapidly than the other subunits during M phase arrest by colcemid treatment and recovered after release from the arrest. Gel filtration chromatography or native (nondenaturing) PAGE analysis followed by immunoblotting indicated that the alpha and delta subunit content in the 700- to 900-kDa CCT complex was appreciably lower in the M phase cells than in asynchronous cells. In vivo, the CCT complex of M-phase-arrested cells was found to bind lower amounts of tubulin than that of asynchronous cells. In vitro, the CCT complex of M phase-arrested cells was less active in binding and folding denatured actin than that of asynchronous cells. On the other hand, the CCT complex of asynchronous cells (a mixture of various phases of cell cycle) exhibited lower alpha and delta subunit content and lower chaperone activity than that of S-phase-arrested cells obtained by excess thymidine treatment. In addition, turnover (synthesis and degradation) rates of the alpha and delta subunits in vivo were more rapid than those of most other subunits. These results suggest that the content of alpha and delta subunits of CCT reduces from the complete active complex in S phase cells to incomplete inactive complex in M phase cells.

Stress and molecular chaperones in disease

Stress, a common phenomenon in today's society, is suspected of playing a role in the development of disease. Stressors of various types, psychological, physical, and biological, abound. They occur in the working and social environments, in air, soil, water, food, and medicines. Stressors impact on cells directly or indirectly, cause protein denaturation, and elicit a stress response. This is mediated by stress (heat-shock) genes and proteins, among which are those named molecular chaperones because they assist other proteins to achieve and maintain a functional shape (the native configuration), and to recover it when partially lost due to stress. Denatured proteins tend to aggregate and precipitate. The same occurs with abnormal proteins due to mutations, or to failure of post-transcriptional or post-translational mechanisms. These abnormal proteins need the help of molecular chaperones as much as denatured molecules do, especially during stress. A cell with normal antistress mechanisms, including a complete and functional set of chaperones, may be able to withstand stress if its intensity is not beyond that which will cause irreversible protein damage. There is a certain threshold that normal cells have above which they cannot cope with stress. A cell with an abnormal protein that has an intrinsic tendency to misfold and aggregate is more vulnerable to stress than normal counterparts. Furthermore, these abnormal proteins may precipitate even in the absence of stress and cause diseases named proteinopathies. It is possible that stress contributes to the pathogenesis of proteinopathies by promoting protein aggregation, even in cells that possess a normal chaperoning system. Examples of proteinopathies are age-related degenerative disorders with protein deposits in various tissues, most importantly in the brain where the deposits are associated with neuronal degeneration. It is conceivable that stress enhances the progression of these diseases by facilitating protein unfolding and misfolding, which lead to aggregation and deposition. A number of reports in the last few years have described research aimed at elucidating the role of heatshock proteins, molecular chaperones in particular, in the pathogenesis of neurodegenerative disorders. The findings begin to shed light on the molecular mechanism of protein aggregation and deposition, and of the ensuing cell death. The results also begin to elucidate the role of molecular chaperones in pathogenesis. This is a fascinating area of research with great clinical implications. Although there are already several experimental models for the study of proteinopathies, others should be developed using organisms that are better known now than only a few years ago and that offer unique advantages. Use of these systems and of information available in databases from genome sequencing efforts should boost research in this field. It should be possible in the not-too-distant future to develop therapeutic and preventive means for proteinopathies based on the use of heat-shock protein and molecular chaperone genes and proteins.

Proteasome-dependent degradation of cytosolic chaperonin CCT

The chaperonin containing t-complex polypeptide 1 (CCT) is a heterooligomeric molecular chaperone that assists in the folding of actin, tubulin, and other cytosolic proteins. We show here that degradation of CCT in mammalian cells is inhibited by a proteasome-specific inhibitor, lactacystin. When CCT synthesis was inhibited by growth arrest of cells, the decrease in CCT levels was much slower in the presence of lactacystin than in its absence. Pulse-chase experiments indicated that degradation of CCT is inhibited 2- to 2.5-fold by addition of lactacystin. In addition, CCT degradation rate in ts85 cells that produce thermolabile ubiquitin-activating enzyme E1 was reduced 3-fold at the nonpermissive temperature compared to the degradation at the permissive temperature. These results indicate that the ubiquitin-proteasome system is involved in CCT degradation.

Transcriptional regulation of the mouse cytosolic chaperonin subunit gene Ccta/t-complex polypeptide 1 by selenocysteine tRNA gene transcription activating factor family zinc finger proteins

The chaperonin containing t-complex polypeptide 1 (CCT) is a molecular chaperone assisting in the folding of proteins in eukaryotic cytosol, and the Ccta (encoding the alpha subunit of CCT)/t-complex polypeptide 1 gene encodes the alpha subunit of CCT. We show here that transcription of the mouse Ccta gene is regulated by selenocysteine tRNA gene transcription activating factor (Staf) family zinc-finger transcription factors ZNF143 and ZNF76. Reporter gene assay using HeLa cells indicated that the Ccta gene promoter contains two 18-base pair-long cis-acting elements with similar sequences at -70 and -20 base pairs (designated CCT alpha subunit gene transcription activating element 1 (CAE1) and CAE2, respectively). By yeast one-hybrid screening of CAE1-binding factors, we isolated human ZNF143, which is known to activate transcription of selenocysteine tRNA and small nuclear RNA genes. DNA binding domains of ZNF143 and ZNF76 produced in E. coli recognized CAE1 and CAE2 elements in electrophoretic mobility shift assay. HeLa cell nuclear extract contained a protein that specifically binds to CAE1 and CAE2 and recognized by anti-ZNF143 antibody. Transcription from a minimal Ccta promoter containing CAE2 element in HeLa cells was enhanced by overexpression of full-length ZNF143 and ZNF76 but inhibited by that of their DNA binding domains alone. These results demonstrate that the Staf family proteins control transcription of at least one of the chaperone-encoding genes besides that of tRNA and small nuclear RNA genes. These RNA and chaperone genes are suggested to be coregulated to facilitate synthesis of mature proteins during active cell growth.