Phosphorylation-dependent structural alterations in the small hsp30 chaperone are associated with cellular recovery

Small heat shock proteins (hsps) act as molecular chaperones by preventing the thermal aggregation and unfolding of cellular protein; however, the manner by which cells regulate chaperone activity remains unclear. In the present study, we examined the role of phosphorylation on the chaperone function of the Xenopus small hsp30. Both heat stress and sodium arsenite treatment in A6 cells resulted in a rapid activation of p38alpha and MAPKAPK-2. Surprisingly, the association of MAPKAPK-2 with hsp30 and its subsequent phosphorylation were more prevalent during recovery after heat stress. Treatment of A6 cells with SB203580, an inhibitor of the p38 MAP kinase pathway, resulted in a loss of hsp30 phosphorylation. Phosphorylation resulted in the formation of smaller multimeric hsp30 complexes and resulted in a significant loss of secondary structure. Consequently the phosphorylation-induced structural changes severely compromised the ability of hsp30 to prevent the heat-induced aggregation of citrate synthase and luciferase in vitro. We confirmed that the loss of chaperone activity was coincident with an attenuated binding of phosphorylated hsp30 with target proteins. Our data suggest that phosphorylation may be necessary to regulate the post-heat stress molecular chaperone activity of hsp30.