MODULATION OF CELLULAR THERMORESISTANCE AND ACTIN FILAMENT STABILITY ACCOMPANIES PHOSPHORYLATION-INDUCED CHANGES IN THE OLIGOMERIC STRUCTURE OF HEAT-SHOCK PROTEIN-27

Phosphorylation of heat shock protein 27 (HSP27) can modulate actin fi lament dynamics in response to growth factors. During heat shock, HSP2 7 is phosphorylated at the same sites and by the same protein kinase a s during mitogenic stimulation. This suggests that the same function o f the protein may be activated during growth factor stimulation and th e stress response. To determine the role of HSP27 phosphorylation in t he heat shock response, several stable Chinese hamster cell lines that constitutively express various level of the wild-type HSP27 (HU27 cel ls) or a nonphosphorylatable form of human HSP27 (HU27pm3 cells) were developed. In contrast to HU27 cells, which showed increased survival after heat shock, HU27pm3 cells showed only slightly enhanced survival . Evidence is presented that stabilization of microfilaments is a majo r target of the protective function of HSP27. In the HU27pm3 cells, th e microfilaments were thermosensitized compared with those in the cont rol cells, whereas wild-type HSP27 caused an increased stability of th ese structures in HU27 cells. HU27 but not HU27pm3 cells were highly r esistant to cytochalasin D treatment compared with control cells. More over, in cells treated with cytochalasin D, wild-type HSP27 but not th e phosphorylated form of HSP27 accelerated the reappearance of actin f ilaments. The mutations in human HSP27 had no effect on heat shock-ind uced change in solubility and cellular localization of the protein, in dicating that phosphorylation was not involved in these processes. How ever, induction of HSP27 phosphorylation by stressing agents or mitoge ns caused a reduction in the multimeric size of the wild-type protein, an effect which was not observed with the mutant protein. We propose that early during stress, phosphorylation-induced conformational chang es in the HSP27 oligomers regulate the activity of the protein at the level of microfilament dynamics, resulting in both enhanced stability and accelerated recovery of the filaments. The level of protection pro vided by HSP27 during heat shock may thus represent the contribution o f better maintenance of actin filament integrity to overall cell survi val.