Expression and Phosphorylation of Mammalian Small Heat Shock Proteins
Mammalian small Hsps (sHsps) comprise 7 members found so far, αA- and αB-crystallin, Hsp25/27, Hsp20 (Kato et al. 1994a ), MKBP (Suzuki et al. 1998 ), HspL27 (Lam et al. 1996 ), and cvHSP (Krief et al. 1999 ), all of which contain the α-crystallin domain
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1 Introduction Mammalian small Hsps (sHsps) comprise 7 members found so far, aA- and aB-crystallin, Hsp25/27, Hsp20 (Kato et al. 1994a), MKBP (Suzuki et al. 1998), HspL27 (Lam et al. 1996), and cvHSP (Krief et al. 1999), all of which contain the a -crystallin domain in the carboxy-terminal half of the molecule. Among these sHsps, only aB-crystallin and Hsp27 are stress-inducible. Although the regulation of expression of Hsp27 and aB -crystallin is not yet fully elucidated, it is believed to be mediated similarly to that of the Hsp70 gene by the binding of a heat shock factor (HSF) to the regulatory heat shock element (HSE) (Morimoto et al. 1990) found in the promoter region of their genes (Hickey et al. 1986; Klemenz et al. 1991; Gaestel et al. 1993). However, activat ion of HSE-bind ing activity of HSF does not always result in activation of genes for Hsps (Jurivich et al. 1992; Cotto et al. 1996). In addition to regulation at the level of activation of HSF, stress-induced synthesis of heat shock or stress proteins (Hsps) is also modulated by other transcription factors, such as a cyclic AMP responsive element binding protein (Choi et al. 1991). Steroid hormones, including.estrogen, are known to be potent inducers of Hsp27 in mammalian cells (Edwards et al. 1980) as originally observed in Drosophila (Ireland and Berger 1982; Ireland et al. 1982). The induction of Hsp27 in chicken embryo cells (Edington and Hightower 1990) and of Hsp25 in Ehrlich ascites tumor cells by cisplatin can furthermore be enhanced without changes in transcriptional activity (Edington and Hightower 1990). The expression of Hsp27 and aB -crystallin seems to be regulated by several independent unknown systems. Phosphorylation of serine residues is a common feature of small Hsps. Phosphorylated forms of aA- and aB-crystallins are known to be present in the ocular lens (Voorter et al. 1986; Chiesa et al. 1987) and Hsp27 can be phosphorylated at serine residues in response to stress (Welch 1985).aB- crystallin is also phosphorylated under stress conditions (Ito et al. 1997b) while Hsp20 in vascular smooth muscles is phosphorylated during cyclic nucleotide-
1 Department of Biochemistry, Inst itute for Developmental Research, Aichi Human Service Center, 713-8 Kamiya, Kasugai, Aichi 480-0392, Japan
Prog ress in Molecular and Subcellular Biology, Vol. 28 A.-P. Arr igo and W.E.G. Muller (Eds.) © Spri nger-Verlag Berlin Heidelberg 2002
K. Kato et al.
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dependent vasorelaxation (Beall et al. 1997). In this chapter we describe the modification of the expression of Hsp27 and aB-crystallin with various factors as well as the phosphorylation of Hsp27, Hsp20, and aB-crystallin.
2 Modification of the Expression of Hsp27 and aB-Crystallin When cells are exposed to heat or chemical insult, expression of genes for Hsps is enhanced and the proteins accumulate in cells reaching a peak after about 1O-16h. Such cells exhibit tolerance to additional stress. In the case of heat shock for 15min, responses of Hsps in cultured cells are detected at 43 DC or hig
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