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J. Biol. Chem., Vol. 280, Issue 52, 43209-43217, December 30, 2005

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Identification of the Preprotein Binding Domain of SecA^*

Efrosyni Papanikou, Spyridoula Karamanou, Catherine Baud, Miriam Frank, Giorgos Sianidis, Dimitra Keramisanou, Charalampos G. Kalodimos, Andreas Kuhn¶, and Anastassios Economou1

From the Institute of Molecular Biology and Biotechnology, F.O.R.T.H. and Department of Biology, University of Crete, P.O. Box 1527, GR-711 10 Iraklio, Crete, Greece, Chemistry Department, Rutgers University, Newark, New Jersey 07102, and ^¶Institute of Microbiology and Molecular Biology, University of Hohenheim D-70593 Stuttgart, Germany

SecA, the preprotein translocase ATPase, has a helicase DEAD motor. To catalyze protein translocation, SecA possesses two additional flexible domains absent from other helicases. Here we demonstrate that one of these "specificity domains" is a preprotein binding domain (PBD). PBD is essential for viability and protein translocation. PBD mutations do not abrogate the basal enzymatic properties of SecA (nucleotide binding and hydrolysis), nor do they prevent SecA binding to the SecYEG protein conducting channel. However, SecA PBD mutants fail to load preproteins onto SecYEG, and their translocation ATPase activity does not become stimulated by preproteins. Bulb and Stem, the two sterically proximal PBD substructures, are physically separable and have distinct roles. Stem binds signal peptides, whereas the Bulb binds mature preprotein regions as short as 25 amino acids. Binding of signal or mature region peptides or full-length preproteins causes distinct conformational changes to PBD and to the DEAD motor. We propose that (a) PBD is a preprotein receptor and a physical bridge connecting bound preproteins to the DEAD motor, and (b) preproteins control the ATPase cycle via PBD.

Received for publication, September 12, 2005 , and in revised form, October 12, 2005.

^* This work was supported by European Union Grants RTN1-1999-00149 QLRT-2000-00122, and QLK3-CT-2000-00082, Pfizer, Inc., and Greek General Secretariat of Research Grants 01AKMON46, PENED2001, and PENED03ED623. 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 supplemental Figs. 1-4.

¹ To whom correspondence should be addressed: Institute of Molecular Biology and Biotechnology, F.O.R.T.H. and Dept. of Biology, University of Crete, P.O. Box 1527, GR-711 10, Iraklio, Crete, Greece. Fax/Tel.: 30-2810-391166; E-mail: aeconomo{at}imbb.forth.gr.

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