ALPHA-B-CRYSTALLIN IS A SMALL HEAT-SHOCK PROTEIN

Sequence similarity between alpha-B-crystallin and small heat shock proteins (HSPs) has prompted us to investigate whether alpha-B-crystallin expression is induced by heat shock. Indeed, accumulation of alpha-B-crystallin was detected immunologically in NIH 3T3 cells after incubation at elevated temperatures and after addition of Cd2+ or sodium arsenite to these cells. Two-dimensional gel electrophoresis revealed identity between alpha-B-crystallin from eye lenses and from heat-treated fibroblasts. The promoter of the alpha-B-crystallin gene was fused to the bacterial chloramphenicol acetyltransferase gene and was shown to confer heat inducibility on this reporter gene in transient transfection assays. A perfect heat shock element within the promoter region is likely to mediate this response. Small HSPs and alpha-B-crystallin were shown to share the following two physical properties: (i) they form supramolecular structures with sedimentation values around 17 S and (ii) they are associated with the nucleus at high temperatures and are localized in the cytoplasm under normal conditions. We conclude that alpha-B-crystallin has to be considered a member of the class of small HSPs.