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Thermodynamic analysis of H1 nuclear import: receptor tuning of importinbeta/importin7.

Wohlwend D, Strasser A, Dickmanns A, Doenecke D, Ficner R

Abteilung für Molekulare Strukturbiologie, Institut für Mikrobiologie und Genetik and GZMB, Georg-August-Universität Göttingen, D-37077 Göttingen, Germany.

The nuclear import of H1 linker histones is mediated by a heterodimer of transport receptors, known as importinbeta and importin7. Interestingly, both importins separately interact with H1, but only as a dimer they facilitate the translocation through the nuclear pore. We identified the H1 binding site of importin7, comprising two extended acidic loops near the C terminus of importin7. The analysis of the H1 import complex assembly by means of isothermal titration calorimetry revealed that the formation of a receptor heterodimer in vitro is an enthalpy-driven process, whereas subsequent binding of H1 to the heterodimer is entropy-driven. Furthermore, we show that the importinbeta binding domain of importin7 plays a key role in the activation of importin7 by importinbeta. This process is allosterically regulated by importinbeta and accounts for a specific tuning of the activity of the importinbeta.importin7 heterodimer. The results presented here provide new insights into cellular strategies to even energy balances in nuclear import and point toward a general regulation of importinbeta-related nuclear import processes.

Published 2 April 2007 in J Biol Chem, 282(14): 10707-19.
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