UC Berkeley

Bacteria make a wide variety of organelles to assist in specific cellular functions. How these organelles are assembled is poorly understood. One model for prokaryotic organelle biogenesis is the magnetosome, a lipid-bound compartment that contains a magnetic crystal allowing magnetotactic bacteria (MTB) to navigate along magnetic field lines in their environments. To create magnetosomes, MTB localizes many proteins specifically to the developing magnetosome membrane. How these proteins are sorted is an active area of research. This work explores magnetosome formation and investigates the sorting of a class of magnetosome proteins that are only localized to magnetosomes under magnetite-forming conditions. The first chapter of this dissertation, an unpublished review article, introduces the topic of magnetosome assembly in magnetotactic bacteria. It explores the selection of Magnetospirillum magneticum AMB-1 and Magnetospirillum gryphiswaldense MSR-1 as model species for studying magnetosome formation, including early work that created genetic tools enabling molecular characterization. In addition, studies are described that used these tools to identify proteins critical to magnetosome formation. Chapter 1 also describes a working model of magnetosome biogenesis and identifies areas where more research is needed to fill in our understanding of this complex process. The second chapter of this dissertation, an unpublished primary research article, investigates the localization of magnetite shaping protein Mms6 in AMB-1. Mms6 had previously been shown to localize to magnetosomes after iron is added to iron-starved cells. This change in localization suggested that either new Mms6 was produced and targeted to magnetosomes upon addition of iron, or pre-existing Mms6 was able to relocalize after iron addition. Using pulse-chase analysis combined with microscopy, we determine that pre-synthesized Mms6 in the cytoplasm is relocalized to magnetosomes in response to environmental cues. Our findings identify several magnetosome proteins and Mms6 protein domains critical to this dynamic localization behavior. The third chapter of this dissertation, an unpublished primary research article, identifies another protein, MamD, that is also sorted based on environmental conditions. MamD is a magnetosome protein that has been shown to bind magnetite crystal and inhibit magnetosome membrane growth. We use fluorescence microscopy to show that MamD, like Mms6, only localizes to magnetosomes when biomineralization of magnetite crystal is possible. In addition, we show that MamD magnetosome localization requires several of the same magnetosome proteins required for Mms6 localization. Our results suggest that there may be a step in magnetosome assembly where a specific cohort of proteins is conditionally recruited to assist in the nucleation and development of magnetite crystal.