Property and Segregation of Magnetosomes

By: Ledia Gebremedhin

Figure 1. A magnetotactic bacterium with a chain of magnetosomes. Source

I think we have all heard of the word  “Bacteria”, and we associate it with negative things. Off course, bacteria can cause disease. However, viruses can also cause it. Many bacteria do not only coexist with us all the time, but they help us do an astonishing array of useful things like make yogurt, break down some garbage, and even maintain our atmosphere. Bacteria are microscopic and very diverse organisms. Numerous studies confirm that bacteria are the primitive organisms on the earth. They are varied and diverse through their morphology, habitat, and reproduction. They have been found that can live in the temperature above the boiling point, extreme cold, high acid or high salt content, etc. There are different species of bacteria with various and unique characteristics. Magnetotactic Bacteria are one of the species with unique characteristic and segregation of organelle. 

Magnetotactic bacteria are motile, mostly aquatic prokaryotes that oriented and migrated along geomagnetic field lines. This capability is based on intracellular magnetic structures known as Magnetosome, which is a prokaryotic membrane that found in magnetotactic bacteria. They vary in shape from square to rectangular and sort of spiked shape, and they form into chains inside the cell. They consist of either iron oxide Magnetite (Fe3O4) or Greigite (Fe3S4), and this gives a magnetic dipole to cells that allow it to respond to a particular magnetic field.  The proteins present precipitate the iron ions that allowed the protection of chain crystals and magnetic minerals within magnetosomes. This organelle has an incredible structure and purpose to the magnetotactic bacteria. 

The Magnetotactic bacteria Magnetospirillum gryphiswaldense is a gram negative, aquatic, mesophilic, and spiral shaped bacterium. They can carry out a process called Magnetotaxis, which bacteria could orient and migrate along earth’s magnetic field lines, allows this bacteria to orient and align themselves in the way match up with the land. Magnetospirillum is surrounded by a lipid bilayer and usually, contains 15-20 crystals of magnetite. 

Magnetospirillum bacteria live in several aquatic environments and grow in low oxygen. This begs the question of how this organism finds the perfect lower oxygen concentrated spot. The magnetosome will guide primarily and help the aquatic cells to them find the optimum oxygen concentration and function as a type of biological compass needle for the bacteria. Magnetosomes chain has a particular shape with crystal size from 30nm to 100nm, and they align themselves linearly and are anchored to the membrane by the proteins, one of which a bacteria actin is MamK. Magnetotactic bacteria exhibit two distinct behaviors regarding their magnetic orientation.  In the polar motion, the bacteria swim North or South. In the axial motion, the bacteria oscillate back and forth. This mobility and coordination of location and orientation allow for magnetotactic bacteria to travel to areas of enhanced growth and survival, as well as permit the optimization of oxygen levels. Since magnetotactic bacteria live and best survive at the aquatic environments with lower oxygen concentration, magnetotaxis is a useful tool for maintaining fitness and reproducing. 

Segregation and transport of organelle of bacteria is not similar as eukaryotic, segregation of bacteria is different from one to another. Bacteria transport and segregate organelle, but they also possess homologs of eukaryotic cytoskeletal proteins. It is a highly conserved process that guarantees that every daughter cell receives a copy of genetic material and plasmid DNA. The Magnetosome is also segregated and transported to the daughter cell. Before the segregation starts, Magnetosomes must be positioned at the mid-cell, cleaved and then separated against the intrachain magnetostatic force.  Many researchers believe that MamK protein and other proteins play an important role during the cell cycle of Magnetospirillum by helping with migration and by moderating the position of magnetosome chain. However, the segregation and placement of magnetosome chain from the pole to the mid-cell have remained difficult to confirm.  This particular study discovered that the MamK filament plays an important role by positioning the MC dynamic pole-to-mid cell.

In Magnetospirillum bacteria, the Magnetosomes chain is positioned at mid-cell, and later localized traversing the division site to be cleaved. Upon mamK deletion, Magnetospirillum cells formed shorter and fragmented magnetosome chains that were no longer recruited to the division site. Based on this observations, it was concluded that newly generated magnetosome sub-chains must undergo a pole-to-mid cell translocation into daughter cells, and MamK was hypothesized to mediate this positioning and migration during the Magnetospirillum cell cycle. However, the pole-to-mid cell movement of the magnetosome chain and the role of MamK in magnetosome chain positioning are yet to be indicated directly. the question is whether the MamK filament may generate the forces required for magnetosome motion and segregation need to be addressed.

One of the motivations for the study of magnetosome segregation is to gain an understanding of the MamK role in the process.  Using advanced electron microscope, they analyzed the basic form of the cell of magnetosome chain and the actin-like mamk filament through the cell cycle.  To evaluate the magnetosome chain localization through the cell cycle, the researchers performed the vivo time-lapse fluorescence imaging of EGFP tagged to the most abundant magnetosome protein. In wild-type (WT) cells, according to the MamC-EGFP fluorescence, the parent single MC chain was located at the mid-cell before the separation. When the magnetosome chain start segregating, the chain positioned at the middle where the segregation takes place, and magnetosome chain starts placed apart from the pole toward the mid-cell into the daughter cell. We have learned that how the wild type cells showed that the magnetosome chain was inherited to daughter cell. Now we can see how the magnetosome chain will segregate while using the mutant strain MamK D161A, this mutant will eliminate the ATPase activity and other actin filaments. In contrast to the wild-type cells, MamK D161A shows different partitioning of the magnetosome chain.  When the segregation takes place, the MamK D161A didn’t pass the Magnetosome chain to the daughter cell center instead exhibited a mislocalization of magnetosome signal next to the cell pole. Instead, after 30 min, a MamC EFGP signal slowly appeared at the end of the pole chain, owing to de novo magnetosome synthesis rather than MC pole-to-mid cell replaced. These can specify that the magnetosome chain was no longer in the MamK D161A strain. Although, a random rearrangement of the magnetosome chains was observed in a minor fraction of Mamk D161A cells.

Figure 2.  The movement of magnetosome chain throughout the cell division cycle. (A) And (B) images represent the wild-type cells positioned and segregation to daughter cells. (A) In vivo time-lapse fluorescent microscopy of MC in the white arrows on shows the where the cytokinesis has been completed for each cell, and the white dot line indicate the MamC-EGFP signal protein progression. (B) Using Kymograph displaying the image of Wild-type cells illustrates the septum and magnetosome chain has been positioned for starting and ending point of the segregation. (C) In vivo time-lapse fluorescent microscopy of MC in the MamK D161A indicates the mislocalized chain at the cell pole. (D) Kymograph displaying the MamC signal of MamK D161A cells, and shows the septum and MC position at starting and ending point. (E) The graph indicates the MC displacement time as a function of time the wild-type cells and MamK D161A strains. Source

Learn more: Toro-Nahuelpan et al. (2016) Segregation of prokaryotic magnetosomes organelles is driven by treadmilling of a dynamic actin-like MamK filament. BMC Biol 14: 88. https://doi.org/10.1186/s12915-016-0290-1