The 3D structure of the outer membrane complex of a type IV secretion system (T4SS) from the bacterium A. tumefaciens was recently solved by the team of Professor Gabriel Waksman from the Institute of Structural and Molecular Biology at Birkbeck college and University College London, using data collected at the PROXIMA 1 beamline. Type IV secretion systems (T4SS) are found in several pathogenic Gram negative bacteria, (for example Bordatella pertussis, Legionella pneumophilia, or the plant bacterium Agrobacterium tumefaciens whose T4SS forms the basis of this study). They form the machinery by which the bacteria inject virulence proteins or DNA into the cells they are infecting. The understanding of the detailed structure of the T4SS (the current structure represents only about half of the full secretion system in A. tumefaciens) may enable the future design of drugs to inhibit their assembly or function. This work is published in the current issue of Nature under the title: Crystal Structure of the Outer Membrane Complex of a Type IV Secretion System : V. Chandran, S. Duquerroy, N. Cronin, J. Navaza and G. Waksman.
| Fig.1 : The outer layer of the Type 4 Secretion System clearly shows the membrane channel used by the bacteria to infect the host cell. |
| Fig.2 : A zoomed view of one of the 14 units making up the outer membrane channel. The 3 protein components are coloured differently. |
To know more
The high resolution crystal structure of the outer membrane part of a T4SS, made up of a complex of 3 proteins, was solved to 2.7 วบ resolution by the method of molecular replacement (MR) using a 18 Å electron microscopy map as search model (usually, in molecular replacement, previously obtained crystal structures are identified in the measured diffraction from a crystal of the whole complex, but here a cryo-electron microscopy map was used : an emerging method to solve crystal structures of large complexes). Attempts to solve the structure ab initio, by single wavelength anomalous diffraction (SAD), using the signal from 112 Selenium atoms, were aided by the unambiguous determination of the crystal "space group" in the MR solution. The structure, of a singular beauty, is made up of 14 copies of the complex of 3 proteins and forms a channel through which virulence proteins may pass from the bacteria to the cell it is infecting. This structure, when compared with earlier work from the same authors using cryo-electron microscopy (reported in Fronzes et al, Science (2009) 323 266 - 268), suggest conformational changes which could regulate the opening and closing of this channel.
