Chapter 52 : Type II Protein Secretion and Twin-Arginine Translocation Promote the Pathogenesis of Legionella pneumophila

Abstract:

Translocation of fully or partially folded proteins across bacterial membranes is a remarkable property of the type II secretion (T2S) system and the twin-arginine translocation (Tat) pathway. This chapter summarizes our current knowledge of the significance of T2S and Tat for Legionella pneumophila. Mutational analysis showed that the Legionella secretion pathway (Lsp) promotes the secretion of at least 11 degradative enzymes, including acid phosphatases, chitinase, zinc-metalloprotease, ribonuclease, mono-, di-, triacylglycerol lipases, phospholipase A (PLA), lysophospholipases A (LPLA) and phospholipases C (PLC). Although all these strategies have been successful, they all have their limitation in defining the complete set of proteins secreted by the Lsp pathway. First, the genes encoding the secreted chitinase, ribonuclease, and tartrate-resistant acid phosphatase have not yet been identified. Second, genetic analysis indicates that some identified exoenzymes do not account for all the corresponding Lsp-dependent activity; e.g., supernatants of the plcA-negative strain retain 50% of the PLC activity of the wild type, indicating that there is more than one Lsp-secreted PLC. Finally, there are probably some additional Lsp-secreted proteins whose function has not been tested for. In conclusion, the Lsp system promotes the secretion of degradative enzymes, growth at low temperature, intracellular replication within amoebae and macrophages, as well as virulence in A/J mice. Moreover, the Tat pathway facilitates secretion of phospholipase C, cytochrome c–dependent respiration, growth in iron-limiting conditions, as well as intracellular replication. Further identification and characterization of Lsp and Tat substrates should therefore enhance our understanding of the pathogenesis of L. pneumophila.