Type III Secretion System

Type III Secretion System (TTSS) is a Type III Secretion System (TTSS) is a mechanism used by bacteria to establish mechanism used by bacteria to establish an infection or symbiotic relationship with an infection or symbiotic relationship with a eukaryotic cell by mediating the injection a eukaryotic cell by mediating the injection of effector proteins into the hosts cells of effector proteins into the hosts cells cytoplasmcytoplasm

TTSS injects (translocates) proteins into TTSS injects (translocates) proteins into the cytosol of eukaryotic cells the cytosol of eukaryotic cells

TTSS’s are activated by bacterial contact TTSS’s are activated by bacterial contact with host cell surfaces with host cell surfaces

Translocated proteins facilitate bacterial Translocated proteins facilitate bacterial pathogenesis by specifically interfering pathogenesis by specifically interfering with host cell signal transduction and other with host cell signal transduction and other cellular processescellular processes E.g. the injected proteins can promote E.g. the injected proteins can promote

bacterial internalization by mammalian cells in bacterial internalization by mammalian cells in Salmonella and Shigella Salmonella and Shigella

Macrophage apoptosis in Yersinia sppMacrophage apoptosis in Yersinia spp

The type III secretion apparatus is The type III secretion apparatus is composed of approximately 20 proteins, composed of approximately 20 proteins, most of which are located in the inner most of which are located in the inner membrane membrane

A subfamily of proteins that include InvG A subfamily of proteins that include InvG from Salmonella, YscC from Yersinia and from Salmonella, YscC from Yersinia and MxiD from Shigella are located in the outer MxiD from Shigella are located in the outer membrane and are required for TTSSmembrane and are required for TTSS These proteins are very similar to Secretins These proteins are very similar to Secretins

that are used to mediate transport across the that are used to mediate transport across the outer membrane of large moleculesouter membrane of large molecules

• This ring of helices is a model of the molecular needle of type III secretion system.

• The model is a combination of the crystal structure of the single subunit and 3D reconstruction of the needle from electron microscopy.

•Most of the inner membrane proteins are homologous to components of the flagellar biosynthesis apparatus

FliK is a protein is used for flagella construction FliK is a protein is used for flagella construction that signals the completion of the hook that signals the completion of the hook component.component.

Once the hook is completed proteins that will Once the hook is completed proteins that will make the flagella components will then be make the flagella components will then be secreted.secreted.

TTSS has a FliK homolog in animal pathogens TTSS has a FliK homolog in animal pathogens such as Salmonella and Shigella. such as Salmonella and Shigella.

The FliK homolog senses when the needle The FliK homolog senses when the needle structure is completed and can send a signal(s) structure is completed and can send a signal(s) to the secretion system so that proteins needed to the secretion system so that proteins needed for the needle secretion will stop being producedfor the needle secretion will stop being produced

Proteins which constitute the type III Proteins which constitute the type III secretion apparatus are conserved among secretion apparatus are conserved among different pathogens. different pathogens.

This suggest that the genes This suggest that the genes

have been spread by have been spread by

horizontal gene transferhorizontal gene transfer Therefore, gene clusters Therefore, gene clusters

are either contained on are either contained on

plasmids or pathogenicity islandsplasmids or pathogenicity islands

•TTSS is coded for by a plasmid called pIB1 in Yersinia spp.

Pathogenicity island of E coli O157:H7 genomes

The effector genes are not linked The effector genes are not linked between species showing that they are between species showing that they are independent of the genes for TTSS independent of the genes for TTSS protein secretionprotein secretion

This allows the bacteria to adapt to host This allows the bacteria to adapt to host countermeasures or to a new hostcountermeasures or to a new host

This is an important process because This is an important process because each different type of bacteria has a each different type of bacteria has a preferential niche, which requires preferential niche, which requires different effector proteinsdifferent effector proteins

Proteins that are predestined for transport Proteins that are predestined for transport through a membrane must be prevented through a membrane must be prevented from assuming their three dimensional from assuming their three dimensional shape prior to transport shape prior to transport

Proteins in their assembled Proteins in their assembled

active form are too large active form are too large

to pass through the small to pass through the small

opening of the needle opening of the needle

structurestructure

The Role of ChaperonesThe Role of Chaperones

Structurally conserved chaperones which Structurally conserved chaperones which specifically bind to individual secreted proteins specifically bind to individual secreted proteins are important in TTSS by preventing premature are important in TTSS by preventing premature interactions of the secreted factors with other interactions of the secreted factors with other proteins.proteins.

Chaperones also ensure presecretory Chaperones also ensure presecretory stabilization and efficient secretionstabilization and efficient secretion

Lack of specific chaperones can reduce the Lack of specific chaperones can reduce the secretion of the protein due to degradation in secretion of the protein due to degradation in the bacteria cytoplasmthe bacteria cytoplasm E.g. In shigella the IpgC chaperone binds to IpaB E.g. In shigella the IpgC chaperone binds to IpaB

and IpaC and inhibits premature association and and IpaC and inhibits premature association and degradation of the IpaBC protein complex before it degradation of the IpaBC protein complex before it is secretedis secreted

Chaperones cap the region required Chaperones cap the region required for translocation to prevent for translocation to prevent premature interaction with other premature interaction with other proteins in TTSS apparatus or from proteins in TTSS apparatus or from self-aggregation prior to secretionself-aggregation prior to secretion

The region of the protein that is bound to The region of the protein that is bound to the chaperone is unfolded and resistant the chaperone is unfolded and resistant to proteolysis to proteolysis

The unbound C terminus remains active The unbound C terminus remains active

Not all proteins require a chaperoneNot all proteins require a chaperone However, proteins with chaperones However, proteins with chaperones

are secreted more rapidly are secreted more rapidly If the protein has no chaperone it is If the protein has no chaperone it is

generally not as efficient as those that do generally not as efficient as those that do have a chaperone associated with ithave a chaperone associated with it

Summary PointsSummary Points TTSS uses a needle like structure to move TTSS uses a needle like structure to move

proteins from the bacteria to the eukaryotic proteins from the bacteria to the eukaryotic host across all three membraneshost across all three membranes

Many of the proteins that form TTSS are Many of the proteins that form TTSS are homologous to those found in bacteria flagellahomologous to those found in bacteria flagella Show support that flagella and TTSS are formed in Show support that flagella and TTSS are formed in

a similar fashiona similar fashion Genes that code for TTSS are highly Genes that code for TTSS are highly

conserved between different species; however conserved between different species; however the genes for effector proteins are different in the genes for effector proteins are different in each specieseach species Shows that the proteins that form TTSS are Shows that the proteins that form TTSS are

independent from effector proteinsindependent from effector proteins Chaperones prevent degradation of effector Chaperones prevent degradation of effector

proteins, early interaction with other proteins, proteins, early interaction with other proteins, and ensure a higher rate of translocationand ensure a higher rate of translocation