title: Structure and function of the XpsE N-terminal domain, an essential component of the Xanthomonas campestris type II secretion system.
abstract: 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 (XpsEN), in two conformational states. By mediating interaction between XpsE and the cytoplasmic membrane protein XpsL, XpsEN anchors XpsE to the membrane-associated secretion complex to allow the coupling between ATP utilization and exoprotein secretion. The structure of XpsEN observed in crystal form P4(3)2(1)2 is composed of a 90-residue alpha/beta 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 I4(1)22, 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 P4(3)2(1)2 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.
authors.affiliation: Institute of Biochemistry, College of Life Sciences, National Chung Hsing University, Taichung City 402, Taiwan.
authors.forename: Yeh
authors.initials: Y
authors.lastname: Chen
authors.forename: Sheng-Jie
authors.initials: SJ
authors.lastname: Shiue
authors.forename: Chia-Wen
authors.initials: CW
authors.lastname: Huang
authors.forename: Jiun-Li
authors.initials: JL
authors.lastname: Chang
authors.forename: Yi-Ling
authors.initials: YL
authors.lastname: Chien
authors.forename: Nien-Tai
authors.initials: NT
authors.lastname: Hu
authors.forename: Nei-Li
authors.initials: NL
authors.lastname: Chan
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000502:physiology*; D018360:Crystallography, X-Ray; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000502:physiology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D016959:Xanthomonas campestris / Q000201:enzymology / Q000378:metabolism*; D000516:alpha-Amylases / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; C071567:general secretion pathway protein, Xanthomonas campestris; D000516:alpha-Amylases