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Fig.1 Protein regulation model of the type III secretion apparatus in Salmonella spp.(from Dr. J. E. Gal¨¢n's laboratory).
Inner membrane proteins: InvA, SpaP, SpaQ, SpaR, SpaS.
Putative associatedinner membrane proteins: InvA, InvE.
Outer membrane proteins: InvG, PrgH, PrgK.
Chaperone: SicA.
Putative chaperone: InvI.
Secreted proteins involved in secretion: InvJ, SpaO.
Secreted proteins with a putative effector function (target proteins): SipA, SipB, SipC, SipD, SptP.
Some of the target proteins could form part of a supramolecular structure.
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| Introduction |
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The Database of Type III Secretion System (DTTSS) was founded in September, 2004. It contains type III secretion apparatus of gram-negative bacteria and flagellar biosynthesis apparatus of most bacteria.
The type III secretion apparatus is composed of approximately 20 proteins,
most of which are located in the inner membrane, and type III secretion
requires a cytoplasmic, probably membrane-associated ATPase. Interestingly,
most of the inner membrane proteins are homologous to components of the
flagellar biosynthesis apparatus of both gram-negative and gram-positive
bacteria, while an outer membrane protein of the type III secretion apparatus
is homologous to PulD, the outer membrane secretin of the type II secretion
pathway. Although type III secretion does not include distinct periplasmic
intermediates of the secreted proteins, transport through the inner membrane
is genetically separable from secretion through the outer membrane, since a
mutant of the outer membrane PulD homolog of P. syringae was shown to
accumulate considerable amounts of a secreted protein in the periplasm. As in
type I and type II secretion, the genes encoding the type III secretion
apparatus are clustered.
As in type I secretion, the proteins secreted via the type III pathway are
not subjected to amino-terminal processing during secretion. The signal for
secretion has long been thought to reside within the amino-terminal 15 to 20
aa of the secreted proteins, since this region is necessary for secretion and
suffices to direct the secretion of hybrid fusion proteins. However, the
amino-terminal sequences of proteins secreted via the type III pathway do not
share any recognizable structural similarities that could function as a
common secretion signal, and exhaustive mutational analysis of some secreted
proteins has revealed a high degree of tolerance for sequence changes within
the amino terminus without loss of secretion. Therefore, it has recently been
proposed that the secretion signal resides in the 59 region of the mRNA which
encodes the secreted proteins. Interestingly, the secreted proteins require
small cytoplasmic proteins with chaperone functions to protect the secreted
factors from premature interaction with other components of the secretion
system. In contrast to type I secretion, which is a true secretory system in
that the secreted enzymes are active in the extracellular space, type III
secretion systems appear to be dedicated machineries for the translocation of
pathogenicity proteins into the cytosol of eukaryotic cells. Accordingly,
protein secretion¡ªat least in some cases¡ªis regulated by contact with the
surface of a target cell. In accordance with the homology of the type III
secretion apparatus to flagellar biosynthesis factors, some type III
secretion systems assemble supermolecular structures on the bacterial
surface, which could be involved in protein translocation into eukaryotic
cells.
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