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FtsZ is necessary for septum formation
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- The septal ring (Z-ring) is a complex of several proteins coded by fts genes of E. coli
that forms at the mid-point of the cell. It gives rise to the septum at
cell division. The first of the proteins to be incorporated is FtsZ,
which gave rise to the original name of the Z-ring.
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The product of ftsZ is required for septum formation at preexisting sites.
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FtsZ is a GTPase that forms a ring on the inside of the bacterial envelope. It is connected to other cytoskeletal components.
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The gene ftsZ plays a central role in division. Mutations in ftsZ
block septum formation and generate filaments. Overexpression induces
minicells, by causing an increased number of septation events per unit
cell mass. ftsZ mutants act at stages varying from the
displacement of the periseptal annuli to septal morphogenesis. FtsZ is
therefore required for usage of preexisting sites for septum formation,
but does not itself affect the formation of the periseptal annuli or
their localization.
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Figure 13.29
Immunofluorescence with an antibody against FtsZ shows
that it is localized at the mid-cell. Photograph kindly provided by
William Margolin (see 3076).
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FtsZ functions at an early stage of
septum formation. Early in the division cycle, FtsZ is localized
throughout the cytoplasm. As the cell elongates and begins to constrict
in the middle, FtsZ becomes localized in a ring around the
circumference (3077). The structure is sometimes called the
Z-ring. Figure 13.29 shows that it lies in the position of the mid-center annulus of Figure 13.25.
The formation of the Z-ring is the rate-limiting step in septum
formation. In a typical division cycle, it forms in the center of cell
1-5 min after division, remains for 15 min, and then quickly constricts
to pinch the cell into two.
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The structure of FtsZ resembles tubulin, suggesting that assembly of
the ring could resemble the formation of
microtubules in eukaryotic cells. FtsZ has GTPase activity, and GTP cleavage is
used to support the oligomerization of FtsZ monomers into
the ring structure. The Z-ring is a dynamic structure, in which there is continuous
exchange of subunits with a cytoplasmic pool (3078; for reveiw see 4526).
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Two other proteins needed for division, ZipA and FtsA, interact directly and independently
with FtsZ (3079).
ZipA is an integral membrane protein, located in the inner bacterial
membrane. It provides the means for linking FtsZ to the membrane. FtsA
is a cytosolic protein, but is often found associated with the
membrane. The Z-ring can form in the absence of either ZipA or FtsA,
but cannot form if both are absent (3080). This suggests that
they have overlapping roles in stabilizing the Z-ring, and perhaps in linking it to the membrane.
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The products of several other fts genes join the Z-ring in a defined order after FtsA
has been incorporated (for review see 123; 982). They are all transmembrane proteins.
The final structure is sometimes called the septal ring.
It consists of a multiprotein complex that is presumed to have the
ability to constrict the membrane. One of the last components to be
incorporated into the septal ring is FtsW, which is a protein belonging
to the SEDS family ftsW is expressed as part of an operon with ftsI,
which codes for a transpeptidase (also called PBP3 for
penicillin-binding protein 3), a membrane-bound protein that has its
catalytic site in the periplasm. FtsW is responsible for incorporating
FtsI into the septal ring (3081). This suggests a model for
septum formation in which the transpeptidase activity then causes the
peptidoglycan to grow inward, thus pushing the inner membrane and
pulling the outer membrane.
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Figure 13.30
Immunofluorescence with antibodies against the Arabidopsis
proteins FtsZ1 and FtsZ2 show that they are localized at the mid point
of the chloroplast (top panel). The bright field image (lower panel)
shows the outline of the chloroplast more clearly. Photograph kindly
provided by Katherine Osteryoung (see 3075).
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FtsZ is the major cytoskeletal component of septation. It is common in bacteria, and is found also in
chloroplasts. Figure 13.30 shows the localization of the plant homologues to a ring at the
mid-point of the chloroplast. Chloroplasts also have other genes
related to the bacterial division genes. Consistent with the common
evolutionary origins of bacteria and chloroplasts, the apparatus for
division seems generally to have been conserved.
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Mitochondria, which also share an
evolutionary origin with bacteria, usually do not have FtsZ. Instead,
they use a variant of the protein dynamin, which is involved in
pinching off vesicles from membranes of eukaryotic cytoplasm (see Different types of coated vesicles
exist in each pathway). This functions from the outside of the organelle, squeezing the membrane to
generate a constriction.
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The common feature, then, in the division
of bacteria, chloroplasts, and mitochondria is the use of a
cytoskeletal protein that forms a ring round the organelle, and either
pulls or pushes the membrane to form a constriction.
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Last Revised on December 10, 2004
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123 Lutkenhaus, J. and Addinall, S. G.
(1997).
Bacterial cell division and the Z ring.
Annu. Rev. Biochem. 66, 93-116.
PubMed Journal
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982 Rothfield, L., Justice, S. and Garcia-Lara, J.
(1999).
Bacterial cell division.
Annu. Rev. Genet. 33, 423-438.
PubMed Journal
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3077 Bi, E. F. and Lutkenhaus, J.
(1991).
FtsZ ring structure associated with division in Escherichia coli.
Nature 354, 161-164.
PubMed Journal
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3078 Stricker, J., Maddox, P., Salmon, E. D., and Erickson, H. P.
(2002).
Rapid assembly dynamics of the Escherichia coli FtsZ-ring demonstrated by fluorescence recovery after photobleaching.
Proc. Natl. Acad. Sci. USA 99, 3171-3175.
PubMed Journal
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3079 Hale, C. A. and de Boer, P. A. (1997).
Direct binding of FtsZ to ZipA, an essential component of the
septal ring structure that mediates cell division in E. coli.
Cell 88, 175-185. PubMed Journal
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3080 Pichoff, S. and Lutkenhaus, J.
(2002).
Unique and overlapping roles for ZipA and FtsA in septal ring assembly in Escherichia coli.
EMBO J. 21, 685-693.
PubMed Journal
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3081 Mercer, K. L., Mercer, K. L., and Mercer, K. L.
(2002).
The E. coli cell division protein FtsW is required to recruit its cognate transpeptidase, FtsI (PBP3), to the division site.
J. Bacteriol. 184, 904-912.
PubMed Journal
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4526 Romberg, L. and Levin, P. A.
(2003).
Assembly dynamics of the bacterial cell division protein FTSZ: poised at the edge of stability.
Annu. Rev. Microbiol. 57, 125-154.
PubMed
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© Jones and Bartlett Publishers (2007)
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