Characterization of Inactive and Stress-Induced Active Forms of the Transcription Factor HSF1: An analysis at the cellular level

Vujanac, Milos (2000). Characterization of Inactive and Stress-Induced Active Forms of the Transcription Factor HSF1: An analysis at the cellular level. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000fcfc

Abstract

In mammalian cells, the heat shock response is mediated by the heat shock transcription fact or HSF1 that forms active, DNA- binding homotrimers during temperature stress. The subcellular localization of the inactive form of HSF1 has been of gr eat interest for the potential implications in signaling in this system.
I have detected the inactive form of HSF1 mostly in the nucleus of diverse mammalian cell lines. However , I have found that HSF1 is not confined to the nucleus , but continuously shuttles between the nucleus and the cytoplasm at a minimum rate of 1 molecule sec^-1. A possible link of shuttling with the functional state of HSF1 is suggested by the observation that the shuttling cycle is discontinued during mild heat stress and resumes promptly during stress relaxation . A similar block of nuclear export is observed for deregulated mutants of HSF1 that trimerize at 37 °C, suggesting that the trimerization step inhibits an export activity.
By mutational analysis I showed that HSF1 contains an unusual bipartite nuclear localization signal. Ongoing experiments are defining sequence requirements for nuclear export, most probably on a pathway distinct from Exportin-1 as judged from the refractoriness of nuclear export of HSF1 to Leptomycin B.
I discuss a possible role of shuttling in compartment specific modifications of HSF1 or associations with co-factors.