Type IV secretion in Gram-negative and Gram-positive bacteria

Abstract

Type IV secretion systems (T4SSs) are versatile multiprotein nanomachines spanning the entire bacterial cell envelope in Gram-negative and Gram-positive bacteria. They play important roles through the contact-dependent secretion of effector molecules into eukaryotic hosts and conjugative transfer of mobile DNA elements as well as contact-independent exchange of DNA with the extracellular milieu. In the last few years, many details on the molecular mechanisms of T4SSs have been elucidated. Exciting structures of T4SS complexes from Escherichia coli plasmids R388 and pKM101, Helicobacter pylori and Legionella pneumophila have been solved. The structure of the F-pilus was also reported and surprisingly revealed a filament composed of pilin subunits in 1:1 stoichiometry with phospholipid molecules. Many new T4SSs have been identified and characterized, underscoring the structural and functional diversity of this secretion superfamily. Complex regulatory circuits also have been shown to control T4SS machine production in response to host cell physiological status or a quorum of bacterial recipient cells in the vicinity. Here, we summarize recent advances in our knowledge of ‘paradigmatic’ and emerging systems, and further explore how new basic insights are aiding in the design of strategies aimed at suppressing T4SS functions in bacterial infections and spread of antimicrobial resistances.

Abbreviated Summary

Type IV secretion systems (T4SSs) are highly sophisticated nanomachines in the cell envelope of many bacteria. They exhibit crucial roles during infection of humans by the secretion of effector proteins, conjugative transfer of DNA and exchange of DNA with the extracellular environment. In this MicroReview, we summarize recent progress on T4SS composition, assembly and structure, and highlight how basic understanding of their functions is aiding in the design of novel strategies for antimicrobial therapies.