Biogenesis and subcellular organization of the magnetosome organelles of magnetotactic bacteria

Abstract

Bacterial cells, like their eukaryotic counterparts, are capable of constructing lipid-based organelles that carry out essential biochemical functions. The magnetosomes of magnetotactic bacteria are one such compartment that is quickly becoming a model for exploring the process of organelle biogenesis in bacteria. Magnetosomes consist of a lipid-bilayer compartment that houses a magnetic crystal. By arranging magnetosomes into chains within the cell, magnetotactic bacteria create an internal compass that is used for navigation along magnetic fields. Over the past decade, a number of studies have elucidated the possible factors involved in the formation of the magnetosome membrane and biomineralization of magnetic minerals. Here, we highlight some of these recent advances with a particular focus on the cell biology of magnetosome formation.

Figure 1

Cell biological features of magnetotactic bacteria. (A) Transmission electron micrograph (TEM) of Magnetospirillum magneticum AMB-1 reveals a linear chain of electron-dense magnetite crystals [44]. (B) Single section of an electron cryotomographic (ECT) image of AMB-1 shows that magnetosome membranes invaginate from the inner membrane before biomineralization [5^••]. (C) Inner membrane invaginations remain even when filled with a mature magnetite crystal [5^••]. (D) ECT images also reveal cytoskeletal filaments flanking the magnetosome chain [5^••]. (E) Magnetosome membranes (yellow), magnetite crystals (orange) and filaments (green) are highlighted in a 3D reconstruction of AMB-1 from an ECT image [5^••].

Figure 2

Magnetosome formation during cell cycle progression. (A) Magnetotactic bacteria increase the number of magnetosomes per cell throughout growth, with the chains centrally located. As the septum forms at the midcell, facilitated by constricting FtsZ rings (green), cytoskeletal filaments flanking the magnetosome chains (yellow) must be separated or stimulated to disassemble. Following cell division, polarly localized magnetosome chains (i) quickly relocalize to the new midcell (ii). (B) The formation of an individual magnetosome is a step-wise process. Magnetosome membrane invagination from the inner cell membrane occurs via the combined actions of MamI, MamL, MamQ, and MamB and other factors. The serine protease MamE is required to properly localize other magnetosome proteins to the compartment. MamK, comprising the cytoskeletal filaments, functions with MamJ and LimJ to coordinate chain organization of the magnetosomes. Protease and putative heme-binding activities of MamE are required for magnetite crystal maturation, with other factors participating in the regulation of crystal number, size, and shape.