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J. Biol. Chem., Vol. 282, Issue 40, 29540-29548, October 5, 2007

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Topology Inversion of SecG Is Essential for Cytosolic SecA-dependent Stimulation of Protein Translocation^*

From the Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan

SecG, a subunit of the protein translocon, undergoes a cycle of topology inversion. To further examine the role of this topology inversion, we analyzed the activity of membrane vesicles carrying a SecG-PhoA fusion protein (SecG-PhoA inverted membrane vesicles (IMVs)). In the absence of externally added SecA, SecG-PhoA IMVs were as active in protein translocation as SecG⁺ IMVs per SecA. Consistent with this observation, insertion of membrane-bound SecA into SecG-PhoA IMVs was normally observed. On the other hand, externally added SecA did not affect the activity of SecG-PhoA IMVs, but it caused >10-fold stimulation of the translocation activity of SecG⁺ IMVs, indicating that the topology inversion of SecG, which cannot occur in SecG-PhoA IMVs, is essential for cytosolic SecA-dependent stimulation of protein translocation. SecG-PhoA IMVs generated a 46-kDa fragment of SecA upon trypsin treatment. The accumulation of this membrane-inserted SecA in the SecG-PhoA IMVs was responsible for the loss of the soluble SecA-dependent stimulation. Moreover, fixation of the inverted SecG topology was found to be dependent on soluble SecA. The dual functions of SecG in protein translocation will be discussed.

Received for publication, June 8, 2007 , and in revised form, August 17, 2007.

^* This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. 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.

¹ To whom correspondence should be addressed. Tel.: 81-3-5841-7831; Fax: 81-3-5841-8464; E-mail: unishiy{at}mail.ecc.u-tokyo.ac.jp.

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