1. Department of Biochemistry and Molecular Biology, University of British Columbia, 2146 Health Sciences Mall, Vancouver, British Columbia, Canada V6T 1Z3
2. University of Washington, Department of Molecular and Cellular Biology, Genome Sciences, HSB K-140, Box 357710, Seattle, Washington 98195, USA
3. Department of Microbiology and Medicine, HSB K-140, Box 357710, Seattle, Washington 98195, USA
4. Michael Smith Laboratories, University of British Columbia, 301–2185 East Mall, Vancouver, British Columbia, Canada V6T 1Z4

Correspondence to: Natalie C. J. Strynadka¹ Correspondence and requests for materials should be addressed to N.C.J.S. (Email: natalie@byron.biochem.ubc.ca).
Coordinates and observed structure factors have been deposited to the Protein Data Bank under accession code 1YJ7.

Type III secretion systems (TTSSs) are multi-protein macromolecular 'machines' that have a central function in the virulence of many Gram-negative pathogens by directly mediating the secretion and translocation of bacterial proteins (termed effectors) into the cytoplasm of eukaryotic cells¹. Most of the 20 unique structural components constituting this secretion apparatus are highly conserved among animal and plant pathogens and are also evolutionarily related to proteins in the flagellar-specific export system. Recent electron microscopy experiments have revealed the gross 'needle-shaped' morphology of the TTSS^{2, 3, 4}, yet a detailed understanding of the structural characteristics and organization of these protein components within the bacterial membranes is lacking. Here we report the 1.8-Å crystal structure of EscJ from enteropathogenic Escherichia coli (EPEC), a member of the YscJ/PrgK family whose oligomerization represents one of the earliest events in TTSS assembly⁵. Crystal packing analysis and molecular modelling indicate that EscJ could form a large 24-subunit 'ring' superstructure with extensive grooves, ridges and electrostatic features. Electron microscopy, labelling and mass spectrometry studies on the orthologous Salmonella typhimurium PrgK within the context of the assembled TTSS support the stoichiometry, membrane association and surface accessibility of the modelled ring. We propose that the YscJ/PrgK protein family functions as an essential molecular platform for TTSS assembly.

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