Identification of a Cytoplasmic Protein Kinase Regulated by IL-1 That Phosphorylates the Small Heat Shock Protein, Hsp27

IL-1 increases phosphorylation of the small heat shock protein (hsp27) in intact cells. This change was also shown both by introducing [gamma-32P]ATP and Mg2+ into MRC-5 fibroblasts permeabilized by LPC after stimulation by IL-1, and by adding the labeled ATP and Mg2+ to cell extracts. Hsp27 phosphorylated in permeabilized cells or cell extracts was shown by 2D electrophoresis to comprise the three forms seen in metabolically labeled cells, suggesting that the physiologically relevant kinase was acting on the substrate in vitro. Mixing of extracts of resting and IL-1-stimulated cells revealed that stimulated cells contained increased levels of kinase activity that phosphorylated substrate hsp27 in the extracts of resting cells. Existence of the activated kinase was confirmed by showing that extracts of IL-1-stimulated cells phosphorylated purified homogeneous hsp27 at a greater rate than those of resting cells. The kinase activity was maximal in cells stimulated with IL-1 for 5 to 10 min, but had declined to the resting level after stimulation for 40 min. Membrane and cytosolic fractions prepared from cell homogenates both contained hsp27 kinase, but the IL-1-dependent increase was associated with the cytosolic fraction. TNF-stimulated cells also contained increased hsp27 kinase activity in the cytosol. The evidence suggests that the cytosolic hsp27 kinase is responsible for the changes in hsp27 phosphorylation induced by the cytokines in intact cells.