MODULATION OF ACTIN MICROFILAMENT DYNAMICS AND FLUID-PHASE PINOCYTOSIS BY PHOSPHORYLATION OF HEAT-SHOCK PROTEIN-27

We recently reported that overexpression of heat shock protein 27 (HSP 27) in rodent fibroblasts increases the stability of stress fibers dur ing hyperthermia and partially prevents actin depolymerization during exposure to cytochalasin D (Lavoie, J. N., Gingras-Breton, G., Tanguay , R. M., and Landry, J. (1993) J. Biol. Chem. 268, 3420-3429). Because HSP27 is a ubiquitous target of phosphorylation upon cell stimulation with a variety of growth factors and agents that affect cellular diff erentiation, we examined the role of HSP27 phosphorylation in regulati ng actin filament dynamics. Here we show that HSP27 is enriched at the leading edge of polarized fibroblasts. HSP27 is localized in lamellip odia and membrane ruffles where most actin polymerization occurs. We d eveloped Chinese hamster cell lines that constitutively overexpressed either human HSP27 or a non-phosphorylatable mutant form of the protei n. Overexpression of HSP27 caused an increased concentration of filame ntous actin (F-actin) at the cell cortex and elevated pinocytotic acti vity. In contrast, overexpression of the non-phosphorylatable mutant f orm of HSP27 reduced cortical F-actin concentration and decreased pino cytosis activity relative to control cells. Mitogenic stimulation of f ibroblasts resulted in a rapid polymerization of submembranous actin f ilaments. HSP27 enhanced growth factor-induced F-actin accumulation, w hereas mutant HSP27 exerted a dominant negative effect and inhibited t his response to growth factors. Thus, HSP27 is a component of a signal transduction pathway that can regulate microfilament dynamics.