The type 2 secretion and type 4 pilus systems of Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 393

CONCLUSIONS
The T2S and T4P systems of E. coli have long been known as important
virulence factors. T2S systems secrete various toxins during pathogenesis
depending upon the specific E. coli variant in question, while T4P are used by
E. coli pathogens for initial adherence to host cells. While in most cases the T2S
apparatus is encoded by genes on the bacterial chromosome, the T4P machin-
ery is generally encoded on a contiguous operon located on a large plasmid.
Both systems share a great deal of structural homology, as both contain a set of
core proteins that form an IM subassembly complex, and an outer membrane
secretin pore. Both systems polymerize a pilus or pilus-like structure as part of
their function. While T2S systems are believed to use the pseudopilus to export
exoproteins, such as toxins, into the extracellular space, the substrate of T4P
systems is the pilus itself. Despite the amount of research published about both
systems, the actual process by which the pseudopilin of the T2S or the pilin of
the T4P is assembled remains poorly understood. As yet, little data have been
published about the immunogenicity of components of either system and the
prospects for vaccine development are hampered by the lack of suitable animal
models. Despite these limitations, both systems hold promise as vaccine or drug
targets given their importance in E. coli virulence .
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