Download - Type 6 Secretion Systems
Type 6 Secretion Systems
Bacterial Wars and Deadly Syringes
What exactly is a T6SS?
Syringe like structure designed to deliver effector proteins
Found in more than 25% of Gram negative bacteria
Delivers them in a cell-contact dependent manner
Used in a defensive manner against competitors
It is also able to inject effectors directly into eukaryotes during an infection
So where did it come from?
Structure and Phage Homology
Show structural and functional homology to the T4 Bacteriophage
Believed to actually be an inverted phage tail
Gene homology with Phage Genes Study by Leiman in 2009 found
homology between phage tail proteins and 3 T6SS components.
Study by Pell in 2009 found similariteis between the Hcp protein of T6SS and a major phage tail protein
Study by Pukatzki in 2007 showed that the VgrG are analogous to the Needle and spike complex of T4 Phage
Delivery of Effectors
The VgrG proteins are located at the distal end of the Hcp tube forming the spike
Following polymerisation of the outer sheath VipA/VipB proteins, the repeating Hcp proteins of the inner tube are ejected outward like a syringe
This is very similar to the mechanism employed by a phage tail
Following this extension effector proteins can be delivered to the target cell.
The exact mechanism however is still to be determined.
Friend or Foe?
Pseudomonas aeruginosa Tae effector proteins Tse1 and Tse3 Tse2 identified as a toxin but
mechanism remains unknown T6SS “duelling” between sister cells
Vibrio cholera effector proteins TseL, VasX and VgrG3
What are these used for? Against rivals of heterologous
bacteria Against sister cells Against eukaryotic cells during an
infection
Immunity to Self Harm
Immunity Determinants Some of the organisms these
have been identified in include Pseudomonas aeruginosa, Burkholderia thailandensis, Vibrio cholera and Serratia marcescens
Protects itself from self intoxication and sister cells by using immunity proteins Tsi1 and Tsi3
Genes encoding effector proteins are located close to ORFs encoding immunity proteins
Research and Future Implications
Pseudomonas T6SS has been shown to play a key role in chronic infections Biofilm formation Quorum Sensing Virulence
2006 Study by Mougous et al found that pulmonary secretions of cystic fibrosis patients contained Hcp1 Pseudomonas infects >80% of Cystic
Fibrosis sufferers A 2014 study by Ma et al reported on the
effctivness of Agrobacterium tumefaxciens T6SS system to kill Pseudomonas within a planta coinfection assay Possible way to turn non pathogenic
bacteria against pathogenic bacteria by using the T6SS machinery
A better understanding of T6SS could offer new antimicrobial treatments to sufferers of chronic diseases such as cystic fibrosis
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