In many bacteria, secretins from the type II secretion system (T2SS) function as outer membrane gated channels that enable passage of folded proteins from the periplasm into the extracellular milieu. Cryo-electron microscopy of the T2SS secretin GspD revealed previously the dodecameric cylindrical architecture of secretins, and crystal structures of periplasmic secretin domains showed a modular domain organization. However, no high-resolution experimental data has as yet been provided about how the entire T2SS secretin or its domains are organized in a cylindrical fashion. Here we present a crystal structure of the N0 domain of the T2SS secretin GspD from enterotoxigenic Escherichia coli containing a helix with 12 subunits per turn. The helix has an outer diameter of ~125. Å and a pitch of only 24. Å which suggests a model of a cylindrical dodecameric N0 ring whose dimensions correspond with the cryo-electron microscopy map of Vibrio cholerae GspD. The N0 domain is known to interact with the HR domain of the inner membrane T2SS protein GspC. When the new N0 ring model is combined with the known N0·HR crystal structure, a dodecameric double-ring of twelve N0-HR heterodimers is obtained. In contrast, the previously observed compact N0-N1 GspD module is not compatible with the N0 ring. Interestingly, a N0-N1 T3SS homolog is compatible with forming a N0-N1 dodecameric ring, due to a different N0-. vs-N1 orientation. This suggests that the dodecameric N0 ring is an important feature of T2SS secretins with periplasmic domains undergoing considerable motions during exoprotein translocation.
|Number of pages||9|
|Journal||Journal of Structural Biology|
|State||Published - Sep 2013|
Bibliographical noteFunding Information:
We thank Stewart Turley and Jonathan Kay for technical support, and Veer Bhatt for stimulating discussions. We also thank the staff of BL12-2 beam line at SSRL for invaluable assistance with data collection. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). This study was funded by National Institute of Health grant AI34501 to WGJH.
- N0 domain
- Type II secretion
- Type IV pilus
ASJC Scopus subject areas
- Structural Biology