Magnetosome biogenesis in Magnetospirillum magneticum AMB-1
The size, shape, and subcellular positioning of organelles in eukaryotes are optimized for their function and can dynamically respond to cellular demands (1, 2). Bacteria make a number of membrane-bound organelles but very little is understood about how the size and positioning of membrane-bound bacterial organelles influence their function (3, 4). This work explores the distinct structural features and dynamic properties that are linked to bacterial organelles. Specifically, this work uses Magnetospirillum magneticum AMB-1 as a model system to explores the formation and positioning of membrane-bound organelle, the magnetosome.
The first chapter of this dissertation, a published review article written in collaboration with a fellow Komeili lab member Nicole Abreu, introduces the topic of compartmentalization and subcellular organization in bacteria (4). Unlike eukaryotic cells, which share a specific set of organelles, bacteria possess a variety of morphologically and functionally distinct organelles that are not shared amongst all bacteria (3). Rather than describe all the bacterial organelles that have been discovered to date, chapter 1 provides vignettes of model organisms that exemplify different modes of membrane remodeling used to achieve compartmentalization. In addition, chapter 1 discusses the mechanisms that position a membrane-bound organelle and a protein-bound organelle from two model systems, magnetotactic bacteria and cyanobacteria.
The second chapter of this dissertation, a published primary research article, uncovers the dynamic nature of a membrane-bound bacterial organelle in the model magnetotactic bacterium. Magnetotactic bacteria make magnetic nanoparticles inside membrane-bound organelles called magnetosomes; however, it is unclear how the magnetosome membrane controls the biomineralization that occurs within this bacterial organelle. In collaboration with Grant Jensen’s lab at Caltech, magnetosome formation was placed under inducible control in Magnetospirillum magneticum AMB-1 (AMB-1) and electron cryo-tomography was used to capture magnetosomes in their near-native state as they form de novo. An inducible system provided the key evidence that magnetosome membranes grow continuously unless they have not properly initiated biomineralization. Our finding that the size of a bacterial organelle impacts its biochemical function is a fundamental advance that impacts our perception of organelle formation in bacteria.
The third chapter of this dissertation (unpublished work) focuses on magnetosome organization and the positioning of newly formed magnetosomes in wildtype AMB-1. In a wildtype AMB-1, the individual magnetosomes of a chain are at a different stages of formation, suggesting that magnetosomes are being made and added to the chain at different times (5). Indeed, maintaining a chain of magnetosomes over multiple cell generations would require the faithful segregation of the magnetosome chain by the mother cell, as well as the correct positioning of new magnetosomes in the daughter cells. To explore the spatial and temporal dynamics of magnetosome positioning, we developed a pulse-chase system to differentially label pre-existing versus newly formed magnetosomes. The preliminary data presented in chapter 3 provide evidence that this pulse-chase system can be used to identify sites of magnetosome biogenesis in relation to the pre-existing magnetosome chain. In addition, this system can be combined with time-lapse imaging to observe how the magnetosome chain is segregated and maintained over multiple cell divisions.