Shifting cells from 30 to 38°C stimulates the degradation of recently synthesized proteins, but not of long-lived proteins, and by 60 min after synthesis, proteins no longer show their response to temperature shift. (A) Proteins were labeled with 35S-Met for 5 min at 30°C to label recently synthesized proteins or 90 min to label long-lived proteins. After washing twice with chase media containing cyclohexamide, the cells were shifted to the indicated temperatures and rates of degradation were measured. The data shown in this and the subsequent experiments represent the means ± SD obtained from three experiments. (B) Cell proteins were labeled with 35S-Met for 5 min, washed, resuspended in chase media, and shifted to 38°C immediately or 30, 60, or 90 min later. The amounts of protein degradation after 1 h were measured. Degradation rates after 2 or 3 h showed similar differences as after 1 h (not depicted).
