Heat Shock Protein 27 (HSP27) is a 27 kDa member of a family of proteins whose expression and function are stimulated by heat shock and other stress stimuli. A major function of these proteins is to serve as chaperones that bind to and stabilize the active conformation of other proteins. HSP27, along with other members of the small HSP group, possesses a C-terminal Alpha-crystalline homology domain. HSP27 is localized to the cytoplasm of unstressed cells but can redistribute to the nucleus in response to stress, where it may function to stabilize DNA and/or the nuclear membrane. Cytoplasmic HSP27 exists in multiple complexes. One complex consists of HSP27, Akt (PKB), MAPKAP-kinase 2, and p38 MAPK. The presence of HSP27 in this complex is required for Akt activation by stress stimuli. Another complex consists of HSP27 and the IKK complex. HSP27 is also an actin capping protein that binds to the barbed (growing) ends of actin filaments, thereby inhibiting filament extension. Phosphorylation of HSP27 on serine 82 by MAPKAP-kinase 2 leads to HSP27 dissociation from the Akt/MAPKAP-kinase 2/p38 complex and from actin filaments, and stimulates HSP27 binding to the IKK complex.
