Molecular chaperones and the ubiquitin-proteasome pathway are known to participate in the quality control of proteins in cells. In this study, we examined the responses of small heat shock proteins to proteasome inhibitors to clarify their roles under conditions where misfolded proteins are abnormally accumulated. HSP27 and αB-crystallin accumulated in both soluble and, more prominently, insoluble fractions after exposure to MG-132, a proteasome inhibitor. Enhanced expression of mRNAs for HSP27 and αB-crystallin was observed, suggesting transcriptional activation. Phosphorylation of HSP27 and αB-crystallin in cells treated with MG-132 was enhanced concomitantly with activation of p38 and p44/42 MAP kinase pathways. Immunofluorescence analysis revealed that exposure to proteasome inhibitors induced the formation of aggresomes in U373 MG cells, to which HSP27 and αB-crystallin were recruited. However, phosphorylation was not required for this accumulation in aggresomes. Thus, HSP27 and αB-crystallin are increased, phosphorylated and localized in aggresomes when proteasome activity is inhibited. © 2002, The Japanese Biochemical Society.