Saturday, May 20, 2006

"Magnetite biominerlization in bacteria" - Arash Komeili

Thus begins the MATERIALS SESSION and ends the ENERGY SESSION.

Magnetotactic bacteria have organelles called magnetosomes with which they can orient in (geo)magnetic fields. They prefer oxic-anoxic interfaces in their environment. By orienting against the geomagnetic field, they can control for lateral motion and search simply in a vertical direction for the interface.

Magnetite has also been found in protists, fish, and potentially honeybees. Magnetite (Fe3O4) is a single domain magnet, and usually of uniform size and shape within an organism. Applications? "Magnetofossils" can be used as biomarkers; contaminating metals could accumulated within magnetosomes to facilitate bioremidiation; perhaps it could be used for nucleic acid and protein purification; and perhaps most exciting of all, magnetite could be used as a contrast agent for MRI.

How are the magnetosomes built? Magnetosomes are not just a single-domain magnetic crystal - they are also surrounded by a lipid membrane, a membrane with certain unique surface proteins. What are the mechanisms of biominerlization? How are organelles developed in eykaryotic cells (very little is known about the endomembrane system in eukaryotes)? Understanding the mechanisms behind magnetosomes could help us understand.

Two ways to explore the question: High resolution imaging and marking. Use Magnetospirillum sp. AMB-1 in the lab (it's a microaerophile, 4-6 doubling time; magnetite production dependant iron concentration). TEM imaging indicates the magnetosome membrane is present before the crystal forms. Collaborated to conduct Cryo-electron tomography (so awesome!!!). Imaging of magnetosomes seem to indicate they are not actually vesicles, but continuous with the cell membrane - every magnetosome had a membrane neck continuous with the cell membrane visible at some angle. This goes for both empty magnetosomes and those filled with the magnetite crystal.

A 100kb "magnetosome island" has been identified that seems to code for the majority of the magnetosome system. They focused on MamK (MreB homologe, Actin-like protein; homologues involved in cell shape determination, plasmid segregation). The structures of MamK and ParM have both been solved. Fillaments can be identified running along the magnetosome chain, and it is hypothesized that MamK is involved in keeping the chain organized; when it is deleted, the chains become broken and disorganized.

Komeili finished by showing us a simply awesome 3D visualization of the magnetosome chain from the interior of the cell based on the cryo-electron tomography data. This was an awesome talk. Go watch the stream of it right now!

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