Quantitative proteomic analysis reveals formation of an EscL-EscQ-EscN type III complex in enteropathogenic Escherichia coli Academic Article uri icon

abstract

  • We characterized Orf5 and SepQ, two type III secretion (T3S) system proteins in enteropathogenic Escherichia coli, and showed that they are essential for T3S, associated with the bacterial membrane, and interact with EscN. Our findings suggest that Orf5 and SepQ are homologs of YscL and YscQ from Yersinia, respectively. Enteropathogenic Escherichia coli (EPEC) belongs to a family of bacterial pathogens that colonize the gut epithelium via formation of distinct histopathological attaching and effacing (A/E) lesions (15). The A/E phenotype is linked to the locus of enterocyte effacement (LEE), the pathogenicity island of EPEC that encodes components of a type III secretion system (T3SS), transcriptional regulators, chaperones, and effector proteins (4, 12). The T3SS is a multiprotein complex that transports effector proteins into target cells. The overall structure of the type III secretion (T3S) apparatus is conserved among different pathogens and resembles the flagellar apparatus. The T3S apparatus consists of three ring structures that transverse the bacterial cytoplasm (C ring), inner membrane (inner ring), and outer membrane (outer ring), joined together by a cylinder (inner rod) and linked to a needle and a filament that reach the host cell membrane to form a pore (translocators) (23, 28–31). The cytoplasmic ring is assumed to consist of an oligomer of a protein from the YscQ/FliN family. This assumption is based on the crystal structures of FliN and HrcQ B , which were previously shown to form doughnut-shaped tetramers that conform in size and shape to electron microscopy (EM) images of the C ring (5, 26). Although PSI-BLAST searches failed to find any YscQ homolog among the EPEC LEE-encoded proteins, a search of the NCBI conserved-domain database (18) identified an FliN domain in the C terminus of the LEE-encoded SepQ protein (25). A recent study by Lara-Tejero et al. suggested that SpaO, the Salmonella YscQ homolog, has a critical function that ensures the hierarchy in T3S, as it serves as a sorting platform for effectors and translocators (17). This finding reinforces the importance of identifying the YscQ/SpaO homolog in EPEC. The Yersinia YscQ protein was previously shown to interact

publication date

  • January 1, 2011