Definition of a Bacterial Type IV Secretion Pathway for a DNA Substrate

SCIENCE, 304, 1170-1173, 2004

Speaker: 伍展弘                                   Commentator: 何漣漪老師

Time: 14:00~15:00  10/13/2004                       Place: Room 601

Abstract:

Bacteria use type IV secretion systems (T4SS) for two fundamental objectives related to pathogenesis - genetic exchange and the delivery of effector molecules to target cells (1). Bacterial conjugation systems are a subfamily of the type IV secretion systems and gene acquisition is an important adaptive mechanism that enables pathogens to cope with a changing environment during invasion of the host. In Agrobacterium tumefaciens, VirB/D4 conjugation system is used to deliver oncogenic transfer DNA (T-DNA) and effector proteins to susceptible plant cells, leading to crown gall disease (2). However, the architecture and mechanism of action of the channel mediating DNA transfer across the bacterial cell envelope remains obscure. In this paper, the authors applied a sensitive assay termed transfer DNA immunoprecipitation (TrIP) to identify the interaction of T-DNA and subunits of the VirB/D4 T4SS and further developed a quantitative version of TrIP to compare levels of DNA substrate recovered in the immunoprecipitates (3). With TrIP assay and quantitative TrIP assay, the presence of the secretory apparatus, such as VirE2 and VirB, does not stimulate the T-DNA processing reaction. Furthermore, The VirD4 T4CP recruits the transfer intermediate independently of contributions from the VirB components of the T4SS or the VirE2 protein substrate is proved by the same methods. In addition, several VirB proteins interacting with T-DNA is also detected and the route of the T-DNA translocation is approximately described. The TrIP assay will ultimately present a detailed mechanistic picture of how type IV machines translocate their substrates across the bacterial cell envelope and this assay should also prove highly useful for studies of many other fundamental processes of bacteria and eukaryotes that involve the movement of DNA across biological membranes

References:

1.     E. Cascales, P. J. Christie. (2003) The versatile bacterial type IV secretion systems. Nature Rev. Microbiol. 1, 137

2. A. C. Vergunst et al., (2000) VirB/D4-dependent protein translocation from Agrobacterium into plant cells. Science 290, 979

3.  E. Cascales and P. J. Christie. (2004) Definition of a bacterial Type IV secretion pathway for a DNA substrate. Science 304, 1170