Polymerization of the Actin-Like Protein MamK, Which Is Associated with Magnetosomes

Abstract

The recombinant actin-like protein MamK was purified from Escherichia coli and used as an antigen to generate the anti-MamK antibody. Immunostaining studies showed a linear distribution of MamK in Magnetospirillum magnetotacticum cells and of MamK in association with magnetosomes. Moreover, we demonstrated that MamK polymerizes into filamentous bundles in vitro.Magnetic bacteria synthesize a unique prokaryotic organelle called the magnetosome with which the organisms navigate along the geomagnetic field (1). Magnetosomes consist of membrane-enclosed magnetite crystal particles that are assembled into a straight chain with some structures, such as a magnetosomal matrix, and an interparticle connection (1,6,18). The magnetosome chain is positioned at the center of the cell, along the long axis of the cell in the Magnetospirillum species. A number of genes encoding magnetosome-associated proteins were identified within the unstable genetic island, referred to as the magnetosome island, in the genomes of the Magnetospirillum species and the Magnetococcus sp. MC-1 strain (16). This magnetosome island is essential for magnetosome formation and contains at least three operons (mamAB, mamDC, and mms) (15). MamK, which is encoded by the mamAB operon, is a new member of the bacterial actin homologue (3,6,7,12,17). Recently, networks of the cytoskeleton-like filamentous structures were observed along the magnetosome chains of Magnetospirillum gryphiswaldense, M. magneticum and M. magnetotacticum (6,7,9,14). Komeili et al. demonstrated that the ⌬mamK strain of M. magneticum AMB-1 abolishes the filamentous structure near the magnetosomes; hence, the filamentous structure may be composed of MamK (7). Furthermore, we reported that the green fluorescent protein-fused M. magneticum AMB-1 MamK protein forms a filamentous organization in the cells of Escherichia coli (12). To understand the structure and function of the MamK cytoskeletal filament as well as other bacterial actin homologues, such as MreB and ParM, preparation of the MamK filament in vitro and characterization of the features are required. However, there have been no reports about polymerization of MamK in vitro.In this study, first, we cloned, expressed, and purified M. magnetotacticum MamK from E. coli. Second, the localizations of MamK in the wild-type M. magnetotacticum cell and in the purified magnetosome chain were confirmed, using immunochemical techniques. Finally, we demonstrated for the first time that the recombinant MamK proteins polymerized into filamentous bundles in vitro.Localizations of MamK in the M. magnetotacticum cell and in the purified magnetosomes. The intracellular localization of MamK was examined and compared to that of MreB by using immunofluorescence microscopy (IFM) with the wild-type M. magnetotacticum cell. M. magnetotacticum MS-1 (ATCC 31632) was cultured in a chemically defined liquid medium (2) under microaerobic condition at 25°C in the dark and then harvested at the early stationary phase. The C-terminal Histagged recombinant M....

Deletion of the ftsZ -Like Gene Results in the Production of Superparamagnetic Magnetite Magnetosomes in Magnetospirillum gryphiswaldense

“…3). Under TEM, the polymers appeared as straight, long, filamentous bundles, similar to those of the MamK polymer (51). The MamK cytoskeletal filament has been suggested to contribute to the maintenance of magnetosome organization and function (20,33).…”

Simultaneously Discrete Biomineralization of Magnetite and Tellurium Nanocrystals in Magnetotactic Bacteria

“…Within other microorganisms, internal tellurium crystals are formed close to the cell's periphery, conform to the contour of the cell envelope, and randomly form in the cytoplasmic space (7,29,32,44). This leads to questions of how these nanorods interact with magnetosome components such as the actin-like protein MamK (18,43) and thus questions about the possibility of whether or not these nanorods are forming within magnetosome-like vesicles. However, tellurium crystals surrounded by membranous structure were not observed by TEM analysis of thin-sectioned samples in this study.…”

Formation of magnetite by bacteria and its application

“…MamK nucleates at multiple sites to form long filaments that were confirmed via protein expression in Escherichia coli (Pradel et al 2006). In vitro polymerization of recombinant MamK was examined, and formation of long filamentous bundles was observed ( Taoka et al 2007). MamJ is an acidic protein associated with the filamentous structure, and it directs the assembly of the BacMP chain (Scheffel et al 2006).…”