Content of nonhistone protein in nuclei after hyperthermic treatment.

When nuclei were isolated from Chinese hamster ovary cells after being heated, there was a large increase in the amount of 3H-tryptophan labeled nonhistone protein in the nucleus relative to the whole cell. After 15 min or 30 min of heating at 45.5 degrees C, the nuclear nonhistone protein content increased by 1.6 or 1.8, respectively. In contrast, when the nuclear nonhistone protein content was determined in the intact cell by using autoradiography to quantify 3H-tryptophan labeled protein in the nucleus and cytoplasm in sections of fixed cells, the nuclear nonhistone protein content increased by only 1.14 or 1.28 for 15 or 30 min at 45.5 degrees C, respectively. Therefore, heat does not induce a massive movement of cytoplasmic protein into the nucleus.