Heat stress induces hsc70/nuclear topoisomerase Icomplex formation in vivo: evidence for hsc70-mediated, ATP-independentreactivation in vitro

• Published in: Proceedings of the National Academy of Sciences - PNAS 1994-03, Vol.91 (5), p.1751-1755
• Main Authors: Ciavarra, R P, Goldman, C, Wen, K K, Tedeschi, B, Castora, F J
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.91.5.1751

We previously demonstrated that in murine T cells thermotolerance correlated with heat shock protein 70 (hsp70) synthesis and protection of nuclear type I topoisomerase (topo I). Topo I activity returned to normal levels following heat stress even in cells not rendered thermotolerant by a prior heat shock. Recovery of topo I activity was not dependent on de novo protein synthesis, suggesting that the cell possesses a pathway(s) for refolding this nuclear protein. In this report we demonstrate that topo I and hsc70, the constitutively produced member of the hsp70 family, associated in vivo during heat stress. That this association 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 hsc70 was inefficient relative to a hsc70-containing cell lysate. Protection from heat inactivation as well as reactivation by hsc70 did not require exogenous ATP. Similarly, reactivation by the cell lysate was not inhibited by ADP or a nonhydrolyzable analogue of ATP. Thus, our studies suggest that nuclear topo I complexes with hsc70 during heat stress, 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.