Evolution and Diversity of Prokaryotic Small Heat Shock Proteins
To understand the evolutionary mechanisms that led to the diversification of the various types of heat shock proteins(Hsps) and their functioning in multichaperone networks is a great challenge (Feder and Hofmann 1999 ). Considerable information is alread
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1 Introduction To understand the evolutionary mechanisms that led to the diversification of the various types of heat shock proteins(Hsps) and their functioning in multichaperone networks is a great challenge (Feder and Hofmann 1999). Considerable information is already available on the evolution of the Hsp60 and Hsp70 families (e.g., Gupta 1995; Budin and Philippe 1998; Karlin and Brocchieri 1998;Macario et al. 1999; Archibald et al. 2000;Brocchieri and Karlin 2000). Relativelyless is known about the early evolution of the small heat shock proteins (sHsps), which are considerably more divergent in structure and function than the Hsp60s and Hsp70s. The sHsps are found in bacteria, archaea and eukaryotes. They range in monomer size between 12 and 43kDa, and are characterized by a conserved "a-crystallin" domain of about 80 residues (reviewed in Plesofsky-Vig et al. 1992; Caspers et al. 1995; Waters and Vierling 1999).The N-terminal domains and C-terminal extensions, if present, are not conserved in sequence and length. The sHsps generally form high molecular weight complexes, between 150 and 800kDa in size, although some occur as dimers or tetramers. The only published crystal structure of any sHsp is as yet that of Methanococcus jannaschii Hsp16.5 (Kim et al. 1998). This archaeal sHsp forms a hollow sphere, -120 A in outer diameter, and composed of 24 subunits with an Ig-like ~-sandwich folding. The characteristic property of most sHsps is their ability to suppress the in vitro aggregation of denaturing proteins, while in vivo their expression protects cells during stress (Ehrnsperger et al. 1997b). For detailed information about the broad variety of structural and functional properties of the sHsps we refer to the other chapters in this Volume. It is the purpose of this chapter to describe the evolutionary diversity of prokaryotic sHsps. In terms of sequence divergence, this diversity is broader than that amongst plant or animal sHsps (de long et al. 1998; Waters and
I Depa rtment of Biochemistry, University of Nijmegen, P.O. Box 9101,6500 HB Nijmegen, The Netherlands 2 Centre for Molecular and Biomolecular Informatics, University of Nijmegen, P.O. Box 9101, 6500 HB Nijmegen , The Netherlands
Progress in Molecular and Sub cellular Biology, Vol. 28 A.-P. Arr igo and W.E.G. Miiller (Eds.) © Springer-Verlag Berlin Heidelberg 2002
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Vierling 1999). In fact, both plant and animal sHsps are suggested to be monophyletic groupings, which may have originated separately from different ancestral prokaryotic sHsps. Knowing the evolutionary history of the prokaryotic sHsps, and comparing their properties, may also help to understand the structure-function relationships of eukaryotic sHsps. This will provide an insight in the pathways along which they diversified, and how they acquired their characteristic structural and functional properties. 2
sHsps in Prokaryotes Most prokaryotic sHsps function as chaperone-like proteins in the cytoplasm, but some have become structural components of the spore coat, as
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