Bacillus subtilis actin-like protein MreB influences the positioning of the replication machinery and requires membrane proteins MreC/D and other actin-like proteins for proper localization
Herv Jol Defeu Soufo
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1
Peter L Graumann
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1
0
Institut fur Mikrobiologie, Biologie II, Universitat Freiburg
,
Stefan-Meier-Str. 19, 79104 Freiburg
,
Germany
1
Biochemie
,
Fachbereich Chemie, Hans-Meerwein-Strae
,
Philipps-Universitat Marburg
,
35032 Marburg
,
Germany
Background: Bacterial actin-like proteins have been shown to perform essential functions in several aspects of cellular physiology. They affect cell growth, cell shape, chromosome segregation and polar localization of proteins, and localize as helical filaments underneath the cell membrane. Bacillus subtilis MreB and Mbl have been shown to perform dynamic motor like movements within cells, extending along helical tracks in a time scale of few seconds. Results: In this work, we show that Bacillus subtilis MreB has a dual role, both in the formation of rod cell shape, and in chromosome segregation, however, its function in cell shape is distinct from that of MreC. Additionally, MreB is important for the localization of the replication machinery to the cell centre, which becomes aberrant soon after depletion of MreB. 3D image reconstructions suggest that frequently, MreB filaments consist of several discontinuous helical filaments with varying length. The localization of MreB was abnormal in cells with decondensed chromosomes, as well as during depletion of Mbl, MreBH and of the MreC/MreD proteins, which we show localize to the cell membrane. Thus, proper positioning of MreB filaments depends on and is affected by a variety of factors in the cell. Conclusion: Our data provide genetic and cytological links between MreB and the membrane, as well as with other actin like proteins, and further supports the connection of MreB with the chromosome. The functional dependence on MreB of the localization of the replication machinery suggests that the replisome is not anchored at the cell centre, but is positioned in a dynamic manner.
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Background
Actin provides vital functions as a cytoskeletal component
in eukaryotic and in prokaryotic cells. In eukaryotes, actin
filaments give mechanical strength to cells in form of a
dynamic cytoskeleton, and are structural fibers in muscle
contraction. Additionally, actin proteins have motor like
functions [1-3], most notably in cell migration through
pushing of membranes. Motility receptors turn on WASP
family proteins, which binds to and activate the Arp2/3
complex. The latter induces branching and growth of actin
filaments [3]. In vitro, actin filaments can deform vesicles
and thus push membranes, providing the force to
elongate cellular extensions such as pseudopods [4,5].
Listeria monocytogenes cells move within macrophages
through propelling by actin bundles that extend at only
one pole of the bacterial cells, due to the ActA protein that
is present at one cell pole and that induces rapid
polymerisation of actin. Bacterial cells also possess several
different actin-like proteins [6]. MreB is essential for cell
viability, its depletion leads to a defect in chromosome
segregation, and ultimately to the formation of round
cells, i.e. to loss of rod cell shape. Except for plasmid
encoded ParM protein, which actively partitions plasmids
[7], the true mode of action and regulation of bacterial
actins is still rather unclear.
During the depletion of Bacillus subtilis MreB or of Mbl,
the second actin ortholog, or of Caulobacter crescentus
MreB, origin regions on the chromosomes fail to separate
properly, leading to a severe (or in case of Mbl more
moderate) segregation defect [8,9], likewise to overproduction
of a dominant negative mreB allele in E. coli [10]. It is
unclear, if actin proteins have a direct role, e.g. as an active
segregation motor, or an indirect influence on the
segregation of chromosomes. In support of an active role, MreB
appears to be associated with the nucleoids, in contrast to
Mbl [8], which is thought to be involved in the insertion
of new cell wall material into the growing peptide glycan
layer [11]. Plasmid encoded E. coli ParR protein binds to
a specific cis site on the duplicated plasmids, which are
located close to the cell centre, and induces
polymerisation of the ParM actin homolog [12]. ParM filaments
contain plasmids at their pole ward ends, so two oppositely
orientated ParM filaments appear to push plasmids
towards each cell pole [7]. On the other hand, C. crescentus
MreB has been shown to also affect cell shape and the
localization of cell wall synthesizing proteins [13], and to
play an important role in determining the global polarity
of the cell, i.e. by affecting the localization of proteins to
the cell pole [9]. MreB and Mbl form helical filaments just
underneath the cell membrane [13-16], which in B.
subtilis are highly dynamic. MreB and Mbl move along helical
tracks, with a speed of about 0.1 m/s, providing
potential motor like force [17]. Actin polymerises into a two
stranded right handed helix through addition of
ATPbound actin monomers. Actin movement arises through
gr (...truncated)