The small heat shock proteins (sHsps) from human (Hsp27) and mouse (Hsp25) form large oligomers which can act as molecular chaperones in vitro and protect cells from heat shock and oxidative stress when overexpressed. In addition, mammalian sHsps are rapidly phosphorylated by MAPKAP kinase 2/3 at two or three serine residues in response to various extracellular stresses. Here we analyze the effect of sHsp phosphorylation on its quaternary structure, chaperone function, and protection against oxidative stress. We show that in vitro phosphorylation of recombinant sHsp as well as molecular mimicry of Hsp27 phosphorylation lead to a significant decrease of the oligomeric size. We demonstrate that both phosphorylated sHsps and the triple mutant Hsp27-S15D,S78D,S82D show significantly decreased abilities to act as molecular chaperones suppressing thermal denaturation and facilitating refolding of citrate synthase in vitro. In parallel, Hsp27 and its mutants were analyzed for their ability to confer resistance against oxidative stress when overexpressed in L929 and 13.S.1.24 cells. While wild type Hsp27 confers resistance, the triple mutant S15D,S78D,S82D cannot protect against oxidative stress effectively. These data indicate that large oligomers of sHsps are necessary for chaperone action and resistance against oxidative stress whereas phosphorylation down-regulates these activities by dissociation of sHsp complexes to tetramers.
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