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J. Biol. Chem., Vol. 282, Issue 47, 33977-33984, November 23, 2007

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Functional Characterization of the Atypical Hsp70 Subunit of Yeast Ribosome-associated Complex^*

Charlotte Conz¹, Hendrik Otto¹, Kristin Peisker, Matthias Gautschi^¶, Tina Wölfle, Matthias P. Mayer^||, and Sabine Rospert²

From the Institute of Biochemistry and Molecular Biology, Zentrum für Biochemie und Molekulare Zellforschung (ZBMZ) and the Fakultät für Biologie, University of Freiburg, D-79104 Freiburg, Germany, the ^¶Institut für Molekularbiologie und Biophysik, ETH-Hönggerberg, Schafmattstrasse 20, HPKE 14, 8093 Zürich, Switzerland, and the ^||Zentrum für Molekulare Biologie, University of Heidelberg (ZMBH), D-69120 Heidelberg, Germany

Eukaryotic ribosomes carry a stable chaperone complex termed ribosome-associated complex consisting of the J-domain protein Zuo1 and the Hsp70 Ssz1. Zuo1 and Ssz1 together with the Hsp70 homolog Ssb1/2 form a functional triad involved in translation and early polypeptide folding processes. Strains lacking one of these components display slow growth, cold sensitivity, and defects in translational fidelity. Ssz1 diverges from canonical Hsp70s insofar that neither the ability to hydrolyze ATP nor binding to peptide substrates is essential in vivo. The exact role within the chaperone triad and whether or not Ssz1 can hydrolyze ATP has remained unclear. We now find that Ssz1 is not an ATPase in vitro, and even its ability to bind ATP is dispensable in vivo. Furthermore, Ssz1 function was independent of ribosome-associated complex formation, indicating that Ssz1 is not merely a structural scaffold for Zuo1. Finally, Ssz1 function in vivo was inactivated when both nucleotide binding and Zuo1 interaction via the C-terminal domain were disrupted in the same mutant. The two domains of this protein thus cooperate in a way that allows for severe interference in either but not in both of them.

Received for publication, August 14, 2007 , and in revised form, September 26, 2007.

^* This work was supported by the Fonds der Chemischen Industrie (to S. R.) and by Sonderforschungsbereich 746 (to S. R.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains a supplemental figure.

¹ Both authors contributed equally to this work.

² To whom correspondence should be addressed: Institute of Biochemistry and Molecular Biology, ZBMZ, Herrmann-Herder-Str. 7, D-79104 Freiburg, Germany, Tel.: 49-761-2035259; Fax: 49-761-2035257, E-mail: sabine.rospert{at}biochemie.uni-freiburg.de.

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