a-Crystallins, versatile stress-proteins

Introduction post-translational modifications [12] of a-crystallins we refer to other recent reviews. a-Crystallins owe their name to the fact that they are major eye lens proteins in vertebrates. They are abun­ dantly present in the lens and have a prominent role in maintaining the transparency and retractile properties of the lens (reviewed in [1-3]). There are two types of related subunits, aA and aB, each of about 20 kDa, of which aB is also expressed at significant levels in many different tissues outside the lens [4], a-Crystallins exist as large homo-or hetcromeric complexes, contain­ ing about 30-40 subunits. Their tertiary and quater­ nary structures are unknown. Initially it was observed that» on ihe basis of sequence homology, a-crystallins belong to the family of small heat shock proteins (hsps) [5], This family is characterized by the presence of an approximately 100 amino acid long conserved domain, often called the a-crystallin domain. a-Crystallins and small hsps arc not only evolutionarily related but they also behave very similarly in many respects. They actu­ ally can form mixed complexes in tissues were both proteins are expressed [6 , 7], a-Crystallins, like oth­ er small hsps, have chaperone-like properties in that they can prevent stress-induced aggregation of pro­ teins [8], Furthermore, mammalian aB-crystallin is stress-inducible, and the presence of a-crystallins in cultured cells leads to an enhanced survival of these cells after a period of stress [9]. aB-Crystallin is also implicated in the pathogenesis of various degenerative diseases. In this mini review we will mainly discuss the struc­ tural and functional aspects of a-crystallins. For gene regulation [10], evolutionary relationships [11] and