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See online version for legend and references. Part I appeared in the January 8 issue of Cell. 294 Cell 140, January 22, 2010 ©2010 Elsevier Inc. DOI 10.1016/j.cell.2010.01.006

SnapShot: Bacterial Appendages IIAna Toste Rego, Rémi Fronzes, and Gabriel WaksmanInstitute of Structural Molecular Biology, Birkbeck and University College London, London WC1E 7HX, UK

This two-part SnapShot depicts the assembly and structure of selected nonflagellar surface appendages from gram-negative bacteria. These include chaperone-usher pili and type IV pili (in part I) and the type III secretion needle and type IV secretion pili (in part II).

Type III Secretion Needle(Left) Type III pili, also called the needle complex or injectisome, is a nanomachine (evolutionarily related to flagellum) involving over 20 proteins. It is composed of a basal body that spans across both outer and inner membranes (OM and IM) and the periplasm (P) and an external needle on the surface of bacteria. The basal body, shown here by the cryo-electron microscopy (cryo-EM) 3D structure of the needle complex of Salmonella typhimurium (gray), consists of two double rings (InvG in red and PrgK/H in dark blue and light orange [Yip et al., 2005; Spreter et al., 2009]) that span the IM and OM linked by a hollow structure (PrgJ in light blue) that crosses the periplasmic space (Marlovits et al., 2006). On the cytoplasmic side other accessory proteins (SpaS in orange and InvK/SpaO in yellow; Deane et al., 2008) as well as ATPases (InvC/SpaL in light green; Zarivach et al., 2007) are in contact with the basal body (InvI/SpaP/SpaQ/InvN/SpaR and InvA are also accessory proteins but are not represented). The external needle is a hollow structure composed of a homopolymer of 100–150 subunits of PrgI (green; Deane et al., 2006). In the distal end of the needle a tip complex composed of SipD (pink; Derewenda et al., 2004) is one of the three translocators (SipB, SipC, SipD) involved in pore formation.

Type IV Secretion Pili(Right) Type IV secretion systems are versatile systems that are found in Gram-negative and Gram-positive bacteria and that secrete a wide range of substrates, from single proteins to protein–protein and protein–DNA complexes. The archetypical type IV secretion (T4S) system found in Agrobacterium tumefaciens is composed of 12 proteins named VirB1-B11 and VirD4 (reviewed in Fronzes et al., 2009a). T4S systems include an extracellular pilus composed of a major and a minor subunit named VirB2 and VirB5, respectively. Three ATPases, VirB4, VirB11, and VirD4, power substrate secretion and possibly assist in the assembly of the system. Biochemical and functional data suggest that the inner membrane (IM) channel would be composed of the polytopic membrane protein VirB6 and the bitopic membrane proteins VirB8 and VirB10. At the outer membrane (OM), VirB10 forms the pore that allows the substrate to reach the extracellular milieu. VirB7, VirB9, and VirB10 form the core complex inserted in both bacterial membranes (Fronzes et al., 2009b). The VirB10 C-terminal domain is associated with the VirB9 C-terminal domain and the lipoprotein VirB7 to form the outer membrane complex (Chandran et al., 2009).

Acknowledgments

We thank N.C.J. Strynadka for The EscN hexamer model coordinates and S. Lea for the LcrV cap model coordinates.

RefeRences

Chandran, V., Fronzes, R., Duquerroy, S., Cronin, N., Navaza, J., and Waksman, G. (2009). Crystal structure of the outer membrane complex of a type IV secretion system. Nature 462, 1011–1015.

Deane, J.E., Roversi, P., Cordes, F.S., Johnson, S., Kenjale, R., Daniell, S., Booy, F., Picking, W.D., Picking, W.L., Blocker, A.J., and Lea, S.M. (2006). Molecular model of a type III secretion system needle: Implications for host-cell sensing. Proc. Natl. Acad. Sci. USA 103, 12529–12533.

Deane, J.E., Graham, S.C., Mitchell, E.P., Flot, D., Johnson, S., and Lea, S.M. (2008). Crystal structure of Spa40, the specificity switch for the Shigella flexneri type III secretion system. Mol. Microbiol. 69, 267–276.

Derewenda, U., Mateja, A., Devedjiev, Y., Routzahn, K.M., Evdokimov, A.G., Derewenda, Z.S., and Waugh, D.S. (2004). The structure of Yersinia pestis V-antigen, an essential virulence factor and mediator of immunity against plague. Structure 12, 301–306.

Fronzes, R., Christie, P., and Waksman, G. (2009a). Structural insights into type IV secretion machineries. Nat. Rev. Microbiol. 7, 703–714.

Fronzes, R., Schaefer, E., Saibil, H., Orlova, E., and Waksman, G. (2009b). Structure of type IV secretion core complex. Science 323, 266–268.

Marlovits, T.C., Kubori, T., Lara-Tejero, M., Thomas, D., Unger, V.M., and Galan, J.E. (2006). Assembly of the inner rod determines needle length in the type III secretion injectisome. Nature 441, 637–640.

Spreter, T., Yip, C.K., Sanowar, S., André, I., Kimbrough, T.G., Vuckovic, M., Pfuetzner, R.A., Deng, W., Yu, A.C., Finlay, B.B., et al. (2009). A conserved structural motif mediates forma-tion of the periplasmic rings in the type III secretion system. Nat. Struct. Mol. Biol. 16, 468–476.

Yip, C.K., Kimbrough, T.G., Felise, H.B., Vuckovic, M., Thomas, N.A., Pfuetzner, R.A., Frey, E.A., Finlay, B.B., Miller, S.I., and Strynadka, N.C. (2005). Structural characterization of the molecular platform for type III secretion system assembly. Nature 435, 702–707.

Zarivach, R., Vuckovic, M., Deng, W., Finlay, B.B., and Strynadka, N.C. (2007). Structural analysis of a prototypical ATPase from the type III secretion system. Nat. Struct. Mol. Biol. 14, 131–137.

294.e1 Cell 140, January 22, 2010 ©2010 Elsevier Inc. DOI 10.1016/j.cell.2010.01.006