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Transcript
Type III Secretion
System
 Type III Secretion System (TTSS) is a
mechanism used by bacteria to establish
an infection or symbiotic relationship with
a eukaryotic cell by mediating the injection
of effector proteins into the hosts cells
cytoplasm
 TTSS injects (translocates) proteins into
the cytosol of eukaryotic cells
 TTSS’s are activated by bacterial contact
with host cell surfaces
 Translocated proteins facilitate bacterial
pathogenesis by specifically interfering
with host cell signal transduction and other
cellular processes
 E.g. the injected proteins can promote
bacterial internalization by mammalian cells in
Salmonella and Shigella
 Macrophage apoptosis in Yersinia spp
 The type III secretion apparatus is
composed of approximately 20 proteins,
most of which are located in the inner
membrane
 A subfamily of proteins that include InvG
from Salmonella, YscC from Yersinia and
MxiD from Shigella are located in the outer
membrane and are required for TTSS
 These proteins are very similar to Secretins
that are used to mediate transport across the
outer 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
that signals the completion of the hook
component.
 Once the hook is completed proteins that will
make the flagella components will then be
secreted.
 TTSS has a FliK homolog in animal pathogens
such as Salmonella and Shigella.
 The FliK homolog senses when the needle
structure is completed and can send a signal(s)
to the secretion system so that proteins needed
for the needle secretion will stop being produced
 Proteins which constitute the type III
secretion apparatus are conserved among
different pathogens.
 This suggest that the genes
have been spread by
horizontal gene transfer
 Therefore, gene clusters
are either contained on
plasmids 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
between species showing that they are
independent of the genes for TTSS
protein secretion
 This allows the bacteria to adapt to host
countermeasures or to a new host
 This is an important process because
each different type of bacteria has a
preferential niche, which requires
different effector proteins
 Proteins that are predestined for transport
through a membrane must be prevented
from assuming their three dimensional
shape prior to transport
 Proteins in their assembled
active form are too large
to pass through the small
opening of the needle
structure
The Role of Chaperones
 Structurally conserved chaperones which
specifically bind to individual secreted proteins
are important in TTSS by preventing premature
interactions of the secreted factors with other
proteins.
 Chaperones also ensure presecretory
stabilization and efficient secretion
 Lack of specific chaperones can reduce the
secretion of the protein due to degradation in
the bacteria cytoplasm
 E.g. In shigella the IpgC chaperone binds to IpaB
and IpaC and inhibits premature association and
degradation of the IpaBC protein complex before it
is secreted
 Chaperones cap the region required
for translocation to prevent
premature interaction with other
proteins in TTSS apparatus or from
self-aggregation prior to secretion
 The region of the protein that is bound to
the chaperone is unfolded and resistant
to proteolysis
 The unbound C terminus remains active
 Not all proteins require a chaperone
 However, proteins with chaperones
are secreted more rapidly
 If the protein has no chaperone it is
generally not as efficient as those that do
have a chaperone associated with it
Summary Points
 TTSS uses a needle like structure to move
proteins from the bacteria to the eukaryotic
host across all three membranes
 Many of the proteins that form TTSS are
homologous to those found in bacteria flagella
 Show support that flagella and TTSS are formed in
a similar fashion
 Genes that code for TTSS are highly
conserved between different species; however
the genes for effector proteins are different in
each species
 Shows that the proteins that form TTSS are
independent from effector proteins
 Chaperones prevent degradation of effector
proteins, early interaction with other proteins,
and ensure a higher rate of translocation