Early heat shock proteins in primary thymocytes. Evidence for transcriptional and translational regulation.

Primary isolates of thymic lymphocytes maintained in vitro provide a physiologically well-characterized system in which to study induction of proteins by heat shock; this response is agent-specific and separable from inductions by glucocorticoids or heavy metals (Maytin, E. V., and Young, D. A. J. Biol. Chem. 258, 12718-12722). Here we identify 68 heat shock protein inductions among more than 2,500 individual proteins separated on giant two-dimensional gels and further describe their time course of appearance, sensitivity to cordycepin (3'-deoxyadenosine), and reversibility during recovery. Thirty-one changes are detectable within 1 h. Among these early increases, 20 are inhibitable by cordycepin. However, 11 early changes are not affected by cordycepin; all represent proteins found in relatively low abundance. Five of these inductions are rapidly reversible during recovery from heat shock, in contrast to most other heat shock proteins whose synthesis is maintained or enhanced. One protein identified here appears to be increased by recovery per se. Overall, these results provide evidence for two separate classes of heat shock inductions in normal mammalian cells, i.e. a transcriptionally regulated group, not readily reversible during recovery, and a translationally regulated group in which several inductions rapidly revert to normal during recovery.