**2.2.1 sHSP5**

α-crystallin, a major structural protein of the vertebrate eye lens, is the most intensively studied representarive member of sHSP family. α-crystallin is one of the three major crystallins of the vertebrate eye lens (Ingolia et al, 1982). However it became a major focus of studies since 1982 when Drosophila sHSP were found to share sequence similarities with α-crystallin. Soon after, it was shown that α-crystallin has other functions, defining it as a molecular chaperone – prevents the thermal aggregation of various proteins, including the lens proteins. In the lens α-crystallin exists as a heteropolymer with the

1988;) It participates in the ubiquitination of proteins through its co-chaperones – BAG1 and

The HSP90 family include ATP – dependent chaperones – HSP90α and HSP90β and GRP94. The two isoforms of HSP90 (HSP90α and HSP90β) that are essential to cells are abundantly expressed under normal conditions (Csermely et al, 1998). HSP90 proteins make up 1-2% of the cytosolic proteins and are additionally synthethised during stress. They participate in cell signalling pathways –ligand dependent transcription factors – Glucocorticoid receptor (Nathan et al, 1995); ligand independent transcription factors – Myo-D, tyrosine and serine/threonine kinases (Hartson et al, 1994; Shaknovich et al, 1992). Their chaperone function is almost entirely limited to these transcription factors and signal transducing kinases. HSP90 family members also have anti-apoptotic functions and stimulate the protein

HSP60 is called chaperonin. It is constitutively expressed, found primarily in the mitochondrial matrix, although up to 15% could be cytoplasmically expressed. It is ATPdependent chaperone, protecting the mitochondrial proteins and facilitating the proteolytic degradation of misfolded proteins. The chaperone function of HSP60 is regulated by a cochaperone, known as HSP10 that modulates substrate binding and ATP-ase activity. In the presence of ADP, HSP60 regulates apoptosis, demonstrating both pro- and antiapoptotic

The small heat shock proteins constitute of a diverse family of ubiquituous intracellular proteins (Arrigo et al, 1998). In human ten different sHSP have been described but only a few of them (HSP27, HSP22 and α-Bcrystallin (HSPB5) are true heat shock proteins expressed in response to stress. sHSP are characterized by small molecular weight (12- 43kDa) and a conserved C-terminal domain (the α-crystallin domain). They share the ability to form globular oligomeric structures with molecular masses ranging between 50-800kDa. The dynamic organization of these proteins is essential for performing their biological activity. It depends on their phosphorylated status which is performed by specific signal transduction pathways. It is generally assumed that stress favors the formation of large oligomers associated with unfolded proteins while phosphorylation does the reverse. Large unphosphorylated oligomers of sHSP have greater potentiality to protect cells through their ability to perform chaperone activity. The formation of small phosphorylated oligomers may be required for the binding of unfolded proteins as well as for the recycling of the

α-crystallin, a major structural protein of the vertebrate eye lens, is the most intensively studied representarive member of sHSP family. α-crystallin is one of the three major crystallins of the vertebrate eye lens (Ingolia et al, 1982). However it became a major focus of studies since 1982 when Drosophila sHSP were found to share sequence similarities with α-crystallin. Soon after, it was shown that α-crystallin has other functions, defining it as a molecular chaperone – prevents the thermal aggregation of various proteins, including the lens proteins. In the lens α-crystallin exists as a heteropolymer with the

CHIP. (Meacham et al, 2001; Luders et al, 2000)

triage (Tsubuki et al, 1994; Lewis et al, 2000).

functions (Bukau et al, 1998).

larger ones (Kato et al, 1996).

**2.2.1 sHSP5** 

**2.2 Small heat shock proteins** 

molecular size of approximately 800kDa, having up to 40 subunits from two gene products – αA and αB. αA is encoded by and constitutes of 173 aminoacids, while αB is encoded by and has 175 aminoacids and both share 57% sequence similarity. In contrast to αA, αB is also constitutively expressed in various tissues with high rates of oxidative stress – skeletal muscles, brain, heart, kidney (Lowe, 1992). Its primary sequence can be organized in three distinct structural regions: an α-crystallin domain of 90 amino-acids in length which is conserved among all sHSP and flanked by an N- and C – terminal domains of variable length and sequence. The conserved α-crystallin domain spans residues 68-148. It is has seven strands, organized in two sheets. The N - terminal domain is highly variable and influences subunit oligomerization and chaperone-like activity, whereas the C-terminal extension stabilizes the global structure and enhances protein/substrate complex (Sun et al, 1997; Bhatacharyya et al, 2002) αB-crystallin is a major structural protein of human lenses that belongs to the family of small heat-shock proteins. It has auto-kinase activity and participates in intracellular architecture and membrane stabilisation (Nicole et al, 2002; Wang K, Spector A, 1996). It acts as molecular chaperone and stabilises proteins in large soluble aggregates in the cytoplasm. The cytoplasmic expression of αB-crystallin is also responsible for the regulation of cyclin-D1 ubiquitination (Liu et al, 2006) and inhibition of pro-apoptotic proteins such as caspase-3, p53, Bax and Bclxs (Mao et al, 2004; Lin et al, 2007)
