The Scientist

Scientists in Germany have identified cytoskeletal structures and a gene that anchor magnetic crystals inside magnetobacteria, organisms that navigate Earth's magnetic field. The findings, which appeared online Sunday in Nature, could help elucidate other poorly understood biomineralization processes, such as those producing magnetic nanoparticles in higher organisms, senior author Dirk Schüler at the Max Planck Institute for Marine Microbiology in Bremen, Germany, told The Scientist.

Magnetobacteria employ organelles known as magnetosomes, magnetite crystals enclosed in the membrane and arranged in chains that behave like compass needles. Higher organisms such as salmon and homing pigeons also possess magnetosome chains resembling those in bacteria. Recently, Schüler and his colleagues identified a cluster of at least 25 -- and possibly up to 100 -- genes in magnetobacterium Magnetospirillum gryphiswaldense apparently involved in magnetite biomineralization and magnetosome formation.

During the current study, the researchers deleted mamJ, which encodes for a protein consisting of many acidic amino acids and a repetitive domain structure, reminiscent of proteins controlling biomineralization in bones, teeth, otoliths and other structures. They found that the size, number and shape of magnetosomes were not affected by the deletion. Still, transmission electron microscopy revealed the magnetosomes were no longer arranged in straight chains, instead collapsing into agglomerates.

Fluorescence microscopy and tagging MamJ with enhanced green fluorescent protein (EGFP) confirmed MamJ was linked to magnetosome vesicles, and revealed MamJ also links to a cytoskeletal structure that runs like a clothesline throughout the entire cell. Cryoelectron tomography showed this structure is made of a network of roughly four nanometer-thick filaments and is found in both wild-type and mutant mamJ cells, suggesting MamJ does not form this filament.

"We knew magnetosomes had to be aligning along some physical structure, but this is the first time we've seen the expression of a specific protein tied with magnetosome alignment," Radu Popa at the University of Southern California in Los Angeles, who did not participate in this study, told The Scientist.

In mamJ mutants, immature and empty vesicles are scattered throughout the cytoplasm and mature magnetosomes clump up. In wild-type cells, magnetosome vesicles are mostly arranged along the filamentous structure. Using transmission electron microscopy, the researchers found immature magnetosomes only clustered together once their magnetite crystals grew in size and number. The researchers suggest MamJ connects empty vesicles to the filamentous structure, and that magnetite crystals then grow within the vesicles.

"Nobody dreamed the Mam cluster was also involved with the arrangement of magnetosomes," Popa said. The fact that genes for both magnetosome formation and arrangement seem clustered together "suggests in the future we can cut out this package and express it in other bacteria that are easier to work with and potentially have the entire system, which is great news," Popa said.

"Biomineralization in general across all domains of life is not very well understood," Katrina Edwards at Woods Hole Oceanographic Institution, who did not participate in this study, told The Scientist. "If we can learn through this how magnetosomes are formed in bacteria, it could help in understanding magnetosome formation and principles of biomineralization in higher organisms," Schüler said.

Future experiments could knock out other magnetosome genes to learn their function, Richard Frankel at California Polytechnic State University in San Luis Obispo, also not a co-author, told The Scientist. For instance, he noted mamJ is co-transcribed with mamK, which encodes an actin-like protein, and he suspected mamK could help assemble the filamentous structure.

Edwards added that magnetobacteria are a very diverse group, and "we'd like to see how ubiquitous this mechanism the researchers describe is among magnetobacteria, or whether different bacteria evolve their own pathways."