Hsp27

From Wikipedia, the free encyclopedia

Hsp27 is a chaperone of the sHsp – small heat shock protein - group among ubiquitin, α-crystallin, Hsp20 and others. The common functions of sHsps are chaperone activity, thermotolerance, inhibition of apoptosis, regulation of cell development and cell differentiation and they take part in signal transduction.

Contents 1 Structure of sHsps 2 Oligomerization 3 Cellular Localization 4 Functions 5 References

[edit] Structure of sHsps

sHsps have some structural features in common: Very characteristic is a homologue 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 et al. 1998, Van Monfort et al. 2001].

The N-terminus consists 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 et al. 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 et al. 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 et al. 2005] and in Hsp25Δ18 [Lindner et al. 2000].

[edit] Oligomerization

The N-terminus with its WD/EPF-region is essential for the development of high molecular oligomers [Haslbeck et al. 2002, Theriault et al. 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 [Gusev et al. 2002], which was shown with the proteins MjHSP16.5 from Methanocaldococcus jannaschii [Kim et al. 1998] and wheat Hsp16.9 [Van Montfort et al. 2001]. 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 [Theriault et al. 2004, Ehrnsperger et al. 1999, Rogalla et al. 1999]. The oligomerization depends on the physiology of the cells, the phosphorylation status 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 kinase cascade by differentiating agents, mitogens, inflammatory cytokines such as TNFα and IL-1β, hydrogen peroxide and other oxidants [Garrido et al. 2000], leads to the activation of MAPKAP kinases 2 and 3 which directly phosphorylate mammalian sHsps [Rogalla et al. 1999]. The phosphorylation plays an important role for the formation of oligomers in exponentially growing cells in vitro, but the oligomerization in tumor cells growing in vivo or growing at confluence in vitro is dependent on cell-cell contact, but not on the phosphorylation status [Bruey et al. 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 [Gusev et al. 2002]. Therefore it is clear, that a formation of large aggregates takes place under heat shock [Ehrnsperger et al. 1999].

[edit] Cellular Localization

Hsp27 appears in many cell types, especially all types of muscle cells. 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. Probably it is essential for development in general: knock-out mice do not survive.

An affinity of high expression levels of different phosphorylated Hsp27 species and muscle/neurodegenerative diseases and various cancers was observed [Sarto et al. 2000]. High expression levels possibly are in inverse relation with cell proliferation, metastasis, and resistance to chemotherapy [Vargas-Roig et al. 1997]. High levels of Hsp27 were also found in sera of breast cancer patients [Rui et al. 2003]; therefore Hsp27 could be a potential diagnostical marker.

[edit] Functions

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 Hsp70-complex.

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 [Sarto et al. 2004]. The phosphorylated form of Hsp27 inhibits Daxx apoptotic protein and prevents the association of Daxx with Fas and Ask1 [Charette et al. 2000].

A well documented function of Hsp27 is the interaction with actin and 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 [Sarto et al. 2004].

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-κB pathway, that controls a lot of processes, such as cell growth and inflammatory and stress responses [Parcellier et al. 2003]. The cytoprotective properties of Hsp27 result from its ability to modulate reactive oxygen species and to raise glutathione levels.

Probably Hsp27 – among other chaperones – is involved in the process of cell differentiation [Arrigo et al. 2005]. Changes of Hsp27 levels were observed in Ehrlich ascite cells, embryonic stem cells, normal B-cells, B-lymphoma cells, osteoblasts, keratinocytes etc. 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.

[edit] References

• 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.
• 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.
• 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.
• 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.
• Garrido C. Size matters: of the small HSP27 and its large oligomers. Cell Death Differ. 9(5):483-5. 2002.
• 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.
• Haslbeck M. sHsps and their role in the chaperone network. Cell Mol Life Sci. 59(10):1649-57. 2002
• Kim KK, Kim R, Kim S. Crystal structure of a small heat shock protein. Nature. 394(6693):595-9. 1998.
• Lelj-Garolla B, Mauk AG. Self-association of a small heat shock protein. J Mol Biol. 345(3):631-42. 2005.
• 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.
• 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.
• 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.
• 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.
• 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.
• Sarto C, Binz PA, Mocarelli P. Heat shock proteins in human cancer. Electrophoresis. 21(6):1218-26. 2000.
• 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.
• 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.
• 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.