Hsp27 is a chaperone of the
sHsp(small heat shock protein) group among ubiquitin, α- crystallin, Hsp20and others. The common functions of sHsps are chaperone activity, thermotolerance, inhibition of apoptosis, regulation of cell development, and cell differentiation. They also take part in signal transduction.
tructure of sHsps
sHsps have some structural features in common: Very characteristic is a homologous and highly conserved amino acid sequence, the so-called α-crystallin-domain at the
C-terminus. These sequences consist of 80 to 100 residues with a homology between 20% and 60% and form β-sheets, which are important for the formation of stable dimers.Kim KK, Kim R, Kim S. Crystal structure of a small heat shock protein. "Nature". 394(6693):595-9. 1998.] Van Montfort R, Slingsby C, Vierling E. Structure and function of the small heat shock protein/alpha-crystallin family of molecular chaperones. "Adv Protein Chem". 59:105-56. 2001.]
N-terminusconsists of a less conserved region, the so-called WD/EPF domain, followed by a short variable sequence with a rather conservative site near the C-terminus of this domain. The C-terminal part of the sHsps consists of the above mentioned α-crystallin domain, followed by a variable sequence with high motility and flexibility.Gusev NB, Bogatcheva NV, Marston SB. Structure and properties of small heat shock proteins (sHsp) and their interaction with cytoskeleton proteins. "Biochemistry (Mosc)". 67(5):511-9. 2002.]
This C-terminal tail appears in many mammalian sHsps (e.g. mouse Hsp25, αA-crystallin) and has no homology. It is highly flexible and polar because of its negative charges.Liao JH, Lee JS, Chiou SH. C-terminal lysine truncation increases thermostability and enhances chaperone-like function of porcine αB-crystallin. "Biochem Biophys Res Commun". 297(2):309-16. 2002.] Probably it functions as a mediator of solubility for hydrophobic sHsps and it stabilizes the protein and protein/substrate complexes. This was shown by elimination of the C-terminal tail in Hsp27Δ182-205 Lelj-Garolla B, Mauk AG. Self-association of a small heat shock protein. "J Mol Biol". 345(3):631-42. 2005.] and in Hsp25Δ18.Lindner RA, Carver JA, Ehrnsperger M, Buchner J, Esposito G, Behlke J, Lutsch G, Kotlyarov A, Gaestel M. Mouse Hsp25, a small shock protein. The role of its C-terminal extension in oligomerization and chaperone action. "Eur J Biochem". 267(7):1923-32. 2000.]
The N-terminus with its WD/EPF-region is essential for the development of high molecular
oligomers,Haslbeck M. sHsps and their role in the chaperone network. "Cell Mol Life Sci". 59(10):1649-57. 2002.] Theriault JR, Lambert H, Chavez-Zobel AT, Charest G, Lavigne P, Landry J. Essential role of the NH2-terminal WD/EPF motif in the phosphorylation-activated protective function of mammalian Hsp27. "J Biol Chem". 279(22):23463-71. 2004.] which exclusively have chaperone activity "in vitro". Hsp27-oligomers probably consist of stable dimers, which are formed by two α-crystallin-domains of neighbouring monomers, which was shown with the proteins MjHSP16.5 from [http://microbewiki.kenyon.edu/mediawiki-1.6.6/index.php/Methanococcus Methanocaldococcus jannaschii] and wheat Hsp16.9. The stable dimers aggregate to tetramers and finally form unstable oligomers.
The oligomerization of Hsp27 is a dynamic process: There is a balance between stable dimers respectively tetramers and instable oligomers (up to 800 kDa) consisting of 16 to 32 subunits and a high exchange rate of subunits.Ehrnsperger M, Lilie H, Gaestel M, Buchner J. The dynamics of Hsp25 quaternary structure. Structure and function of different oligomeric species. "J Biol Chem". 274(21):14867-74. 1999.] Rogalla T, Ehrnsperger M, Preville X, Kotlyarov A, Lutsch G, Ducasse C, Paul C, Wieske M, Arrigo AP, Buchner J, Gaestel M. Regulation of Hsp27 oligomerization, chaperone function, and protective activity against oxidative stress/tumor necrosis factor alpha by phosphorylation. "J Biol Chem". 274(27):18947-56. 1999.] The oligomerization depends on the physiology of the cells, the
phosphorylationstatus of Hsp27 and the exposure to stress. Stress induces an increase of expression (after hours) and phosphorylation (after several minutes) of Hsp27. Stimulation of the p38 MAP kinasecascade by differentiating agents, mitogens, inflammatory cytokinessuch as TNFα and IL-1β, hydrogen peroxideand other oxidants,Garrido C. Size matters: of the small HSP27 and its large oligomers. "Cell Death Differ". 9(5):483-5. 2002.] leads to the activation of MAPKAP kinases 2 and 3 which directly phosphorylate mammalian sHsps. The phosphorylation plays an important role for the formation of oligomers in exponentially growing cells "in vitro", but the oligomerization in tumorcells growing "in vivo" or growing at confluence "in vitro" is dependent on cell-cell contact, but not on the phosphorylation status.Bruey JM, Paul C, Fromentin A, Hilpert S, Arrigo AP, Solary E, Garrido C. Differential regulation of HSP27 oligomerization in tumor cells grown in vitro and in vivo. "Oncogene". 19(42):4855-63. 2000.]
In all probability, the oligomerization status is connected with the chaperone activity: aggregates of large oligomers have high chaperone activity, whereas dimers have no chaperone activity. Therefore it is clear, that a formation of large aggregates takes place under heat shock.
Hsp27 appears in many
cell types, especially all types of musclecells. It is located mainly in the cytosol, but also in the perinuclear region, endoplasmatic reticulum, and nucleus. It is overexpressed during different stages of cell differentiation and development. This suggests an essential role for Hsp27 in the differentiation of tissues.
An affinity of high expression levels of different phosphorylated Hsp27 species and muscle/
neurodegenerative diseasesand various cancerswas observed.Sarto C, Binz PA, Mocarelli P. Heat shock proteins in human cancer. "Electrophoresis". 21(6):1218-26. 2000.] High expression levels possibly are in inverse relation with cell proliferation, metastasis, and resistance to chemotherapy.Vargas-Roig LM, Fanelli MA, Lopez LA, Gago FE, Tello O, Aznar JC, Ciocca DR. Heat shock proteins and cell proliferation in human breast cancer biopsy samples. "Cancer Detect Prev". 21(5):441-51. 1997.] High levels of Hsp27 were also found in sera of breast cancerpatients Rui Z, Jian-Guo J, Yuan-Peng T, Hai P, Bing-Gen R. Use of serological proteomic methods to find biomarkers associated with breast cancer. "Proteomics". 3(4):433-9. 2003.] ; therefore Hsp27 could be a potential diagnostical marker.
General functions are the thermotolerance "in vivo", the cytoprotection, and the support of cell survival under stress conditions. Special functions are manifold and complex. "In vitro" it acts as an ATP-independent chaperone by inhibiting protein aggregation and by stabilizing partially denatured proteins, which ensures refolding by the
Hsp27 is also involved in the apoptotic signalling pathway. Hsp27 interacts with the outer mitochondrial membranes and interferes with the activation of
cytochrome c/ Apaf-1/dATP complex and therefore inhibits the activation of procaspase-9. The phosphorylated form of Hsp27 inhibits Daxxapoptotic protein and prevents the association of Daxx with Fas and Ask1.Charette SJ, Lavoie JN, Lambert H, Landry J. Inhibition of Daxx-mediated apoptosis by heat shock protein 27. "Mol Cell Biol". 20(20):7602-12. 2000.]
A well documented function of Hsp27 is the interaction with
actinand intermediate filaments. It prevents the formation of non-covalent filament/filament interactions of the intermediate filaments and protects actin filaments from fragmentation. It also preserves the focal contacts fixed at the cell membrane.
Another function of Hsp27 is the activation of the
proteasome. It speeds up the degradation of irreversibly denatured proteins and junkproteins by binding to ubiquitinated proteins and to the 26S proteasome. Hsp27 enhances the activation of the NF-κBpathway, that controls a lot of processes, such as cell growth and inflammatory and stress responses.Parcellier A, Schmitt E, Gurbuxani S, Seigneurin-Berny D, Pance A, Chantome A, Plenchette S, Khochbin S, Solary E, Garrido C. HSP27 is a ubiquitin-binding protein involved in I-kappaBalpha proteasomal degradation. "Mol Cell Biol". 23(16):5790-802. 2003.] The cytoprotective properties of Hsp27 result from its ability to modulate reactive oxygen speciesand to raise glutathionelevels.
Probably Hsp27 – among other chaperones – is involved in the process of cell differentiation.Arrigo AP. In search of the molecular mechanism by which small stress proteins counteract apoptosis during cellular differentiation. "J Cell Biochem". 94(2):241-6. 2005.] Changes of Hsp27 levels were observed in
Ehrlich ascitecells, embryonic stem cells, normal B-cells, B- lymphomacells, osteoblasts, keratinocytesetc. The upregulation of Hsp27 correlates with the rate of phosphorylation and with an increase of large oligomers. It is possible that Hsp27 plays a crucial role in the termination of growth.
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