Nature of the sHsp Oligomer

Dynamic Nature of the sHsp Oligomer A. Subunit Exchange 

In total these data indicate that sHsp oligomers are in rapid equilibrium with a smaller subassembly, which is not seen under standard analytical conditions. A similar subunit exchange has been reported between dimers of the tetrameric bacterial chaperone SecB (Topping et al., 2001) and monomers of the dimeric ft P 2-cryslallin (Slingsby and 

Bateman, 1994). Coleman and colleagues (1994) have documented rapid exchange between subunits of dimeric ornithine decarboxylase and suggest that this exchange facilitates regulation by antizyme. In contrast, there is no evidence for a rapid equilibrium between subunits of the 14-subunit chaperone GroEL (A. Horwich, personal communication), or in a number of other multisubunit enzymes (Distefano et al., 1990 Perry et al., 1992 Wente and Schachman, 1987). The potential importance of subunit exchange to sHsp function is discussed further below. 

There is an extensive literature on phosphorylation of a-crystallins and ofmammalian sHsps (Gaestel, 2001 Groenen etal, 1994 Kantorowand 

The importance of sHsp phosphorylation for in vivo protection of cell function has been conflicting, potentially reflecting different cellular requirements for the activities of oligomeric and dissociated sHsps in different cell types (Rogalla etal., 1999). The difference in the effect of phosphorylation on chaperone activity in vitro is more difficult to explain. Rogalla and colleagues (1999) report phosphorylation inhibits chaperone activity, while previous reports indicated it had no effect (Knauf et al., 1994). One possibility is that the earlier sHsp preparations were contaminated with phosphatases that removed the phosphates prior to the assay. 

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