Despite their apparent simplicity, bacterial cells are defined by a high degree of subcellular complexity and organization. One of the more dramatic examples of subcellular differentiation is the formation of protein- and lipid-bounded organelles by a variety of bacterial species. One such organelle, the magnetosome, is lipid-bounded and houses the machinery to build nano-sized magnetite crystals. We use magnetosomes as a model to understand the molecular mechanisms of compartmentalization and biomineral production. We are also beginning to explore the ferrosomes in some bacteria, and likely archaea, as a new model organelle.
Genetics of Magnetosome Formation
Magnetotactic bacteria produce magnetosomes through a complex and tightly controlled process. This process is controlled by genes that are typically organized into magnetosome gene clusters or islands. In the Komeili lab, we have developed the tools to understand the molecular basis of magnetosome formation and magnetite biomineralization in the bacterium Magnetospirillum magneticum AMB-1. These advances have helped us to uncover the potential function of a large number of magnetosome genes, including a core set of genes that regulate nearly every step of magnetosome formation, including magnetosome membrane formation, magnetite biomineralization, and magnetosome chain alignment.
While the minimum set of genes required for magnetosomes is known, many questions remain, including:
What are the specific roles for magnetosome genes?
What are the functions of genes in the magnetosome gene cluster that have a subtle or no phenotype when deleted?
Are there genes outside of the magnetosome gene cluster with a role, perhaps indirect, in magnetosome formation?
Current projects in the lab aim to answer these questions using new techniques such as random barcoded transposon-site sequencing in collaboration with researchers in Adam Arkin's lab.
Magnetosome Cell Biology
Proteins necessary for each of the steps of magnetosome formation have been identified in Magnetospirillum magneticum AMB-1, but the mechanisms of these fundamental processes are still poorly understood. While the regulation of magnetosome size has mainly focused on the magnetic crystals, we have found that the membrane surrounding individual crystals may play a greater role than previously expected. In addition, the spacing of magnetosomes and their orientation as a chain along the negatively curved inner membrane is also a complex and tightly regulated process.
Using multiple molecular and biochemical techniques as well as advanced imaging including structured illumination microscopy and cryo-electron tomography, we are addressing the following questions:
How are proteins targeted to magnetosomes?
Which proteins control magnetosome membrane growth?
How are magnetosome chains organized in vivo?
Further research will allow more control over magnetosome formation, the expression of magnetosome synthesis genes in other organisms, and could lead to a greater understanding of the bacterial synthesis of other intracellular compartments.
Magnetotactic bacteria are incredibly diverse and display stunning variety in chain arrangement, crystal morphology, and the quantity of their magnetosomes. We are interested in understanding the basis of this phenotypic diversity by developing other magnetotactic bacteria for genetic and molecular work. In particular, we are working with Desulfovibrio magneticus RS-1, a bacterium that produces tooth-shaped crystals as opposed to the cubo-octahedral crystals of Magnetospirillum magneticum AMB-1. Tooth-shaped magnetite crystals and greigite crystals are only found in deeper branching magnetotactic bacteria and uncovering the mechanism behind the synthesis of these crystals could give us insight into the evolution of magnetosomes.
While genetics have been tricky in Desulfovibrio magneticus RS-1, we have demonstrated that both forward and reverse genetics are possible. In the future, we hope to address the following questions:
Does Desulfovibrio magneticus RS-1 have a unique mode of biomineralization?
Are the functions of magnetosome proteins conserved among magnetotactic bacteria?
Ferrosome Formation & Function
In addition to making magnetosomes, Desulfovibrio magneticus RS-1 makes another type of organelle that stores iron called a ferrosome. Ferrosomes appear to be surrounded by a lipid-like membrane, suggesting that ferrosome formation involves membrane invagination and iron storage via specialized proteins. Using proteomics and genetics, we discovered the genetic basis for ferrosomes and are now pursuing diverse bacteria to address our questions of ferrosome formation.
We are using genetics and fluorescent reporter proteins to investigate the steps of ferrosome formation. With these experiments, we aim to build a foundation for understanding organelle biogenesis in bacteria and archaea.