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Molecular and Cellular Biology, March 2001, p. 1759-1768, Vol. 21, No. 5
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.5.1759-1768.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

The Balance of Nuclear Import and Export Determines the Intracellular Distribution and Function of Tomato Heat Stress Transcription Factor HsfA2

Dirk Heerklotz,1 Pascal Döring,1 Frank Bonzelius,2 Sybille Winkelhaus,1 and Lutz Nover1,*

Department of Molecular Cell Biology1 and Institute of Zoology,2 Biocenter, Goethe-University Frankfurt, Frankfurt am Main, Germany

Received 14 September 2000/Returned for modification 2 November 2000/Accepted 1 December 2000

Tomato heat stress transcription factor HsfA2 is a shuttling protein with dominant cytoplasmic localization as a result of a nuclear import combined with an efficient export. Besides the nuclear localization signal (NLS) adjacent to the oligomerization domain, a C-terminal leucine-rich motif functions as a nuclear export signal (NES). Mutant forms of HsfA2 with a defective or an absent NES are nuclear proteins. The same is true for the wild-type HsfA2 if coexpressed with HsfA1 or in the presence of export inhibitor leptomycin B (LMB). Fusion of the NES domain of HsfA2 to HsfB1, which is a nuclear protein, caused export of the HsfB1-A2NES hybrid protein, and this effect was reversed by the addition of LMB. Due to the lack of background problems, Chinese hamster ovary (CHO) cells represent an excellent system for expression and functional analysis of tomato Hsfs. The results faithfully reflect the situation found in plant cells (tobacco protoplasts). The intriguing role of NLS and NES accessibility for the intracellular distribution of HsfA2 is underlined by the results of heat stress treatments of CHO cells (41°C). Despite the fact that nuclear import and export are not markedly affected, HsfA2 remains completely cytoplasmic at 41°C even in the presence of LMB. The temperature-dependent conformational transition of HsfA2 with shielding of the NLS evidently needs intramolecular interaction between the internal HR-A/B and the C-terminal HR-C regions. It is not observed with the HR oligomerization domain (HR-A/B region) deletion form of HsfA2 or in HsfA2-HsfA1 hetero-oligomers.

* Corresponding author. Mailing address: Department of Molecular Cell Biology, Biocenter N200, 30G, Goethe-University Frankfurt, Marie-Curie-Str. 9, D-60439 Frankfurt, Germany. Phone: (49)69-798-29284. Fax: (49)69-798-29286. E-mail: nover{at}cellbiology.uni-frankfurt.de.

Molecular and Cellular Biology, March 2001, p. 1759-1768, Vol. 21, No. 5
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.5.1759-1768.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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