STRESS-INDUCED AND MITOGEN-INDUCED PHOSPHORYLATION OF THE SMALL HEAT-SHOCK PROTEIN HSP25 BY MAPKAP KINASE 2 IS NOT ESSENTIAL FOR CHAPERONE PROPERTIES AND CELLULAR THERMORESISTANCE

Small heat shock proteins (sHsps) show a very rapid stress- and mitoge n-dependent phosphorylation by MAPKAP kinase 2. Based on this observat ion, phosphorylation of sHsps was thought to play a key role in mediat ing thermoresistance immediately after heat shock, before the increase d synthesis of heat shock proteins becomes relevant. We have analysed the phosphorylation dependence of the chaperone and thermoresistance-m ediating properties of the small heat shock protein Hsp25. Surprisingl y, overexpression of Hsp25 mutants, which are not phosphorylated in th e transfected cells, confers the same thermoresistant phenotype as ove rexpression of wild type Hsp25, which is either mono- or bis-phosphory lated at serine residues 15 and 86 within the cells. Furthermore, in v itro phosphorylated Hsp25 shows the same oligomerization properties an d the same chaperone activity as the nonphosphorylated protein. No dif ferences between phosphorylated and nonphosphorylated Hsp25 are detect ed in preventing thermal aggregation of unfolding proteins and assisti ng refolding of denatured proteins. The results suggest that chaperone properties of the small heat shock proteins contribute to the increas ed cellular thermoresistance in a phosphorylation-independent manner.