HEAT-STRESS INDUCES HSC70 NUCLEAR TOPOISOMERASE-I COMPLEX-FORMATION IN-VIVO - EVIDENCE FOR HSC70-MEDIATED, ATP-INDEPENDENT REACTIVATION IN-VITRO/

We previously demonstrated that in murine T cells thermotolerance corr elated with heat shock protein 70 (hsp70) synthesis and protection of nuclear type I topoisomerase (topo I). Topo I activity returned to nor mal levels following heat stress even in cells not rendered thermotole rant by a prior heat shock. Recovery of topo T activity was not depend ent on de novo protein synthesis, suggesting that the cell possesses a pathway(s) for refolding this nuclear protein. In this report we demo nstrate that topo I and hsc70, the constitutively produced member of t he hsp70 family, associated in vivo during heat stress. That this asso ciation may play a physiologically important role in protecting topo I activity from heat stress was suggested by the observation that hsc70 protected topo I from heat inactivation in vitro. hsc70 but not actin also reactivated previously heat-denatured topo I in a dose-dependent fashion. However, refolding of heat-denatured topo I by purified hsc7 0 was inefficient relative to a hsc70-containing cell lysate. Protecti on from heat inactivation as well as reactivation by hsc70 did not req uire exogenous ATP. Similarly, reactivation by the cell lysate was not inhibited by ADP or a nonhydrolyzable analogue of ATP. Thus, our stud ies suggest that nuclear topo I complexes with hsc70 during heat stres s, which may explain, at least in part, why hsp70 proteins accumulate in the nucleus, particularly the nucleolus. This interaction may limit heat-induced protein damage and/or accelerate restoration of protein function in an ATP-independent reaction.