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J. Biol. Chem., Vol. 280, Issue 51, 42356-42363, December 23, 2005

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Structure and Function of the XpsE N-Terminal Domain, an Essential Component of the Xanthomonas campestris Type II Secretion System^*

From the Institute of Biochemistry, College of Life Sciences, National Chung Hsing University, Taichung City 402, Taiwan

Secretion of fully folded extracellular proteins across the outer membrane of Gram-negative bacteria is mainly assisted by the ATP-dependent type II secretion system (T2SS). Depending on species, 12-15 proteins are usually required for the function of T2SS by forming a trans-envelope multiprotein secretion complex. Here we report crystal structures of an essential component of the Xanthomonas campestris T2SS, the 21-kDa N-terminal domain of cytosolic secretion ATPase XpsE (XpsE_N), in two conformational states. By mediating interaction between XpsE and the cytoplasmic membrane protein XpsL, XpsE_N anchors XpsE to the membrane-associated secretion complex to allow the coupling between ATP utilization and exoprotein secretion. The structure of XpsE_N observed in crystal form P4₃2₁2 is composed of a 90-residue / sandwich core domain capped by a 62-residue N-terminal helical region. The core domain exhibits structural similarity with the NifU-like domain, suggesting that XpsE_N may be involved in the regulation of XpsE ATPase activity. Surprisingly, although a similar core domain structure was observed in crystal form I4₁22, the N-terminal 36 residues of the helical region undergo a large structural rearrangement. Deletion analysis indicates that these residues are required for exoprotein secretion by mediating the XpsE/XpsL interaction. Site-directed mutagenesis study further suggests the more compact conformation observed in the P4₃2₁2 crystal likely represents the XpsL binding-competent state. Based on these findings, we speculate that XpsE might function in T2SS by cycling between two conformational states. As a closely related protein to XpsE, secretion ATPase PilB may function similarly in the type IV pilus assembly.

Received for publication, June 23, 2005 , and in revised form, September 6, 2005.

The atomic coordinates and structure factors (codes 2D27 and 2D28) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported by National Science Council Grants NSC92-2113-M-005-016 (to N.-L. C.) and NSC92-2311-B-005-015 (to N.-T. H.) and Ministry of Education Grants E-91-B-FA05-2-4 (to N.-L. C.) and E-91-B-FA05-1-4 to (to N.-T. H.). 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 may be addressed. Tel.: 886-4-22840468 (ext. 228); Fax: 886-4-22853487; E-mail: nthu{at}nchu.edu.tw.

² To whom correspondence may be addressed. Tel.: 886-4-22840468 (ext. 232); Fax: 886-4-22853487; E-mail: nlchan{at}dragon.nchu.edu.tw.

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