Magnetotactic bacteria have evolved complex subcellular machinery to construct linear chains of magnetite nanocrystals that allow the host cell to sense direction. Each mixed-valent iron nanoparticle is mineralized from soluble iron within a membrane-encapsulated vesicle termed the magnetosome, which serves as a specialized compartment that regulates the iron, redox, and pH environment of the growing mineral. In order to dissect the biological components that control this process, we have carried out genetic and biochemical studies of proteins proposed to function in iron mineralization in Magnetospirillum magneticum AMB-1. As iron biomineralization by magnetotactic bacteria represents a particularly interesting case for understanding how the production of nanomaterials can be programmed at the genetic level, we also apply synthetic biology techniques towards the production of new cellular materials and new cellular functions.
As the production of magnetite requires both the formation of Fe(II) and Fe(III), the redox components of the magnetosome play an essential role in this process. Using genetic complementation studies, we show that the redox cofactors or heme sites of the two putative redox partners, MamP and MamT, are required for magnetite biomineralization in vivo and that removal of one or both sites leads to defects in mineralization. We develop and optimize a heterologous expression method in the E. coli periplasm to cleanly isolate fully heme-loaded MamP for biochemical studies. Spectrochemical redox titrations show that the reduction potential of MamP lies in a different range than other c-type cytochrome involved in either Fe(III) reduction or Fe(II) oxidation. Nonetheless, in vitro mineralization studies with MamP and Fe(II) show that it is able to catalyze the formation of mixed-valent Fe(II)/Fe(III) oxides such as green rust.
Biomineralization also requires lattice-templating proteins that guide the growth of the functional crystalline material. We use in vitro binding and synthesis studies with putative magnetite-templating proteins, the Mms6 family proteins, to show that they are competent to bind and stabilize non-thermodynamically stable faces of magnetite. We also use in vitro iron mineralization to show that the Mms6 family proteins can work together with the redox protein MamP to produce mixed-valent iron oxides from soluble Fe(II) species and to control mineral structure. Further studies with Mms7ct indicate that it and other Mms6 family proteins may play a more significant role in controlling magnetite mineral structure than previously hypothesized. Beyond simple control of size and shape of magnetite, it may also template the crystal lattice of the mineral itself similar to what has been observed with calcium biomineralization, where unstable crystal forms and phases of the mineral are stabilized by interaction with peptides and other macromolecules.
We next set up and begin testing systems to engineer magnetotactic bacteria for the production of new functional materials. We replace the metal-binding C-terminus of native Mms6 family proteins in AMB-1 with peptides known to precipitate metal oxides in vitro under mild conditions. Initial characterization of the behavior of these constructs in AMB-1 has been carried out, although additional experiments are required to test whether they can enable formation of new materials in vivo. We also discuss developing a cellular biosensor based on the formation of a magnetic material in response to an analyte.