The nuclear matrix is a thermolabile cellular structure

Heat shock sensitizes cells to ionizing radiation, cells heated in S phase have increased chromosomal aberrations, and both Hsp27 and Hsp70 translocat e to the nucleus following heat shock, suggesting that the nucleus is a sit e of thermal damage. We show that the nuclear matrix is the most thermolabi le nuclear component. The thermal denaturation profile of the nuclear matri x of Chinese hamster lung V79 cells, determined by differential scanning ca lorimetry (DSC), has at least 2 transitions at T-m = 48 degreesC and 55 deg reesC with an onset temperature of approximately 40 degreesC. The heat abso rbed during these transitions is 1.5 cal/g protein, which is in the range o f enthalpies for protein denaturation. There is a sharp increase in 1-anili nonapthalene-8-sulfonic acid (ANS) fluorescence with T-m = 48 degreesC, ind icating increased exposure of hydrophobic residues at this transition. The T-m = 48 degreesC transition has a similar T-m to those predicted for the c ritical targets for heat-induced clonogenic killing (T-m = 46 degreesC) and thermal radiosensitization (T-m 47 degreesC), suggesting that denaturation of nuclear matrix proteins with T-m = 48 degreesC contribute to these form s of nuclear damage. Following heating at 43 degreesC for 2 hours, Hsc70 bi nds to isolated nuclear matrices and isolated nuclei, probably because of t he increased exposure of hydrophobic domains. In addition, approximately 25 % of exogenous citrate synthase also binds, indicating a general increase i n aggregation of proteins onto the nuclear matrix. We propose that this is the mechanism for increased association of nuclear proteins with the nuclea r matrix observed in nuclei isolated from heat-shocked cells and is a form of indirect thermal damage.