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J. Horwitz; Recent Advances in Our Understanding of the Structure and Function of Alpha-Crystallin . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2075.
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© ARVO (1962-2015); The Authors (2016-present)
In spite of many years of intensive research, the structure of alpha-crystallin is not known. The best glimpse that we have into the quaternary structure of alpha-crystallin comes from the recent work of Kim and his collaborators, where they solved by x-ray crystallographical studies of the three-dimensional structure of the small heat shock protein from Methanococus jannaschii (MjHSP16.5). This 16.5-kDa protein is a member of the small heat shock protein family and contains the core "alpha-crystallin" domain. MjHSP16.5 contains 24 monomers, which are assembled to form a hollow spherical structure. It should be emphasized that there is only 21% sequence identity in the "alpha-crystallin domain" between MjHSP16.5 and alpha A crystallin. In fact, MjHSP16.5 differs drastically in many of its physico-chemical properties to that of alpha-crystallin or HSP27. Among the major differences is the fact that MjHSP16.5 is not polydisperse. It is always found as a unique assembly made up of 24 subunits. In recent experiments that we have performed we have found that there is no subunit exchange at temperature up to 60 degrees centigrade with MjHSP16.5. In addition, MjHSP16.5 is not a good chaperone for most of the target proteins when assayed at temperatures below 60 degrees centigrade. Also MjHSP16.5 is a highly symmetrical molecule; whereas, all evidence to date suggest that alpha-crystallin assemblies lack symmetry. More recently the x-Ray structure of the small heat-shock protein from wheat HSP16.9 was solved. Unlike alpha-crystallin, this protein is a homogeneous dodecamer. The data in the literature suggests that the chaperone function of alpha-crystallin depends on its oligomeric state. However, recently we succeeded to modify alpha-crystallin in such a manner that we get monomeric, dimeric and tetrameric forms of alpha-crystallin in solution. We find that the monomeric form of alpha-crystallin is as efficient chaperone as the native oligomeric form. We also found that the chaperone efficiency of alpha A and alpha B may differ greatly depending on the specific target protein used.
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