ParM

ParM is a prokaryotic actin homologue which provides the force to drive copies of the R1 plasmid to opposite ends of rod shaped bacteria before mitosis.

ParM is a monomer that is encoded in the DNA of the R1 plasmid and manufactured by the host cell’s ribosomes. In the cytoplasm it spontaneously polymerizes forming short strands that either bind to ParR or hydrolyze. ParR stabilizes ParM and prevents it from hydrolyzing. Once bound by ParR at both ends, monomer units continue to attach to the ends of the ParM and the resulting reaction pushes R1 plasmids to opposite ends of the cell.

cite journal
author = Hoischen C.; Busiek, M.; Langowski, J.; Diekmann, S.;
year = 2008
month =
title = Escherichia coli low-copy-number plasmid R1 centromere parC forms a U-shaped complex with its binding protein ParR
journal = Nucleic Acids Research
volume = 36
issue = 2
pages = 607-615
id = 10.1093/nar/gkm672|ISSN 0305-1048
url =
]

Action

In vitro the ParM monomer has been observed polymerizing both with ATP and with GTP, but experiments by Popp et al seem to indicate that the reaction “prefers” GTP and that GTP is the nucleotide that most likely makes the significant contributions in the cell.cite journal
author = Popp, D; Narita
year = 2008
month =
title = Molecular structure of the ParM polymer and the mechanism leading to its nucleotide-driven dynamic instability
journal = Embo Journal
volume = 27
issue = 3
pages = 570-579
id = 10.1038/sj.emboj.7601978|ISSN 0261-4189
] For the remainder of this article GTP will be assumed to be the active nucleotide although many experiments have used ATP instead.

ParM binds and hydrolyzes GTP as it polymerizes. The current dominant belief is that a “cap” of GTP is required at the ends of the ParM polymer strands to prevent them from hydrolyzing. Although GTP is hydrolyzed by the ParM units after attachment, it is believed that the energy that drives the plasmids is derived from the Gibbs free energy of the ParM monomer concentrations, and not the energy released from GTP hydrolysis. The concentrations of ParM monomer and polymer must be kept out of equilibrium at the ends where attachment is occurring for the reaction to proceed regardless of GTP concentrations.

Once the ParM has pushed plasmids to opposite ends of the cell the polymer rapidly hydrolyzes—returning the monomer units to the cytoplasm. cite journal
author = Garner, E.C.; Campbell, C.S.; Weibel, D.B.; Mullins, R.D.
year = 2007
month =
title = Reconstitution of DNA segregation driven by assembly of a prokaryotic actin homolog
journal = Science
volume = 315
issue = 5816
pages = 1270 - 1274
id = 10.1126/SCIENCE.1138527|ISSN 0036-8075
]

tructure

The ParM monomer unit is non-functional before binding a GTP nucleotide. Once the GTP has been bound it can attach to the end of a growing filament. At some point after attachment the ParM hydrolyzes GTP which becomes GDP and remains in the ParM unit as long as the polymer strand remains intact. ParM forms a left-handed helix structure.

cite journal
author = Popp, D; Narita
year = 2008
month =
title = Molecular structure of the ParM polymer and the mechanism leading to its nucleotide-driven dynamic instability
journal = Embo Journal
volume = 27
issue = 3
pages = 570-579
id = 10.1038/sj.emboj.7601978|ISSN 0261-4189
]

A study by Garner and Campbell has suggested that the unit at the end of the ParM strand must have GTP bound to maintain the stability of the polymer. If one of the ends has the GDP bound version the polymer strand hydrolyzes very quickly into its constituent monomer units. This is suggested by their experiment in which they cut growing ParM polymer strands exposing ADP bound ends. Once cut the strands quickly hydrolyzed. cite journal
author = Garner, E.C.; Campbell, C.S.; Weibel, D.B.; Mullins, R.D.
year = 2007
month =
title = Reconstitution of DNA segregation driven by assembly of a prokaryotic actin homolog
journal = Science
volume = 315
issue = 5816
pages = 1270 - 1274
id = 10.1126/SCIENCE.1138527|ISSN 0036-8075
]

Dynamic Instability

Dynamic instability is described as the switching of a polymer between phases of steady elongation and rapid shortening. This process is essential to the function of eukaryotic microtubules. In ParM, dynamic instability “rescue” or the switch from a shortening phase back to the elongation phase has very rarely been observed, and only when the ATP nucleotide is used. Unbound ParM filaments are found with a typical average length of 1.5 – 2 µm, when the ParM monomer concentrations are 2µM or more. The dynamic instability of ParM and other eukaryotic microtubules is believed to be an example of convergent evolution. cite journal
author = Garner, E.C.; Campbell, C.S.; Mullins, R.D.
year = 2004
month =
title = Dynamic instability in a DNA-segregating prokaryotic actin homolog
journal = Science
volume = 306
issue = 5698
pages = 1021-1025
id =
]

ParM spontaneously forms short polymer segments when it is present in the cytoplasm. These segments serve to very efficiently “search” for the R1 plasmids, and also maintains a favorable concentration of ParM monomer units for polymerization. cite journal
author = Popp, D; Narita
year = 2008
month =
title = Molecular structure of the ParM polymer and the mechanism leading to its nucleotide-driven dynamic instability
journal = Embo Journal
volume = 27
issue = 3
pages = 570-579
id = 10.1038/sj.emboj.7601978|ISSN 0261-4189
]

References


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