ParM is a prokaryotic actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 homologue which provides the force to drive copies of the R1 plasmid
R1 plasmid
The R1 Plasmid is a plasmid that was first isolated from Salmonella paratyphi bacteria in 1963.The R1 plasmid imparts multi-drug antibiotic resistance to its host bacteria....

 to opposite ends of rod shaped bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 before mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...


ParM is a monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

 that is encoded in the DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 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
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....



In vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...

 the ParM monomer has been observed polymerizing both with ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 and with GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...

, 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. 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
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...

 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 depolymerizes—returning the monomer units to the cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...



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 subunit as long as the polymer strand remains intact. ParM forms a left-handed helix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...


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 depolymerizes 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.

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 microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

s. 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 eukaryotic microtubules is believed to be an example of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

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.
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