Responses to heat shock of alpha B crystallin and HSP28 in U373 MG human glioma cells.

Responses to heat shock of alpha B crystallin and small heat-shock protein HSP28, which are expressed at considerable levels in human astroglioma U373 MG cells (2-4 micrograms of each per mg soluble protein in confluent cultures), were analysed quantitatively by specific immunoassays. Concentrations of alpha B crystallin and HSP28 in soluble extracts of U373 MG cells decreased to about 50% of original values, with an increase in the insoluble fraction, during heat treatment for 15 min at 45 degrees C. The concentrations of alpha B crystallin and HSP28 increased gradually upon return to 37 degrees C, reaching and then exceeding the control levels within 5 h and 10 h, respectively, after heat shock. During centrifugation on sucrose density gradients both alpha B crystallin and HSP28 in extracts from untreated and heat-treated cells sedimented at the same position, which corresponded to a molecular mass of > 540 kDa. This result suggests that the sizes of aggregates of the two proteins in the cytoplasm are not affected by heat shock. Both alpha B crystallin and HSP28 in an extract of U373 MG cells were trapped on and coeluted from an affinity column prepared with antibodies against alpha B crystallin. These results suggest that the two proteins are also associated in U373 MG cells.