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Phosphorylation of heat shock protein 27 (HSP27) has been suggested to play an important role in the regulation of F-actin dynamics in response to growth factors and stress. Because the microfilament network is one of the earliest targets of oxidative stress and because phosphorylation of HSP27 is strongly induced by reactive oxygen metabolites, we have investigated the role of HSP27 phosphorylation in regulating actin dynamics in response to oxidative stress. Experiments were done in Chinese hamster CCL39 cells overexpressing various levels of the wild-type or a nonphosphorylatable form of human HSP27 (pm3 HSP27). In control cells, both H₂O₂ and menadione induced fragmentation of F-actin, which forms aggregates and patches concentrated around the nucleus. Stable overexpression of wild-type HSP27, but not of pm3 HSP27, conferred resistance against actin fragmentation, suggesting that HSP27 …