期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：47
页码：13396-13401
DOI：10.1073/pnas.1612034113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceSome bacteria contain tiny magnets, so-called magnetosomes, that allow them to swim oriented by the Earths magnetic field. An intracellular filamentous protein, MamK, has previously been shown to align the magnetosomes along a straight line to combine their magnetic moments. Here we show the structure the MamK filaments have at the atomic level. We also show what happens when individual MamK molecules come together to form the filaments, because it is known that filament formation triggers the hydrolysis of the ATP molecule bound to MamK. The structure of monomeric MamK was determined by X-ray crystallography and of the filament by electron cryomicroscopy, image processing, and helical reconstruction. Magnetotactic bacteria produce iron-rich magnetic nanoparticles that are enclosed by membrane invaginations to form magnetosomes so they are able to sense and act upon Earths magnetic field. In Magnetospirillum and other magnetotactic bacteria, to combine their magnetic moments, magnetosomes align along filaments formed by a bacterial actin homolog, MamK. Here, we present the crystal structure of a nonpolymerizing mutant of MamK from Magnetospirillum magneticum AMB-1 at 1.8-[IMG]f1.gif" ALT="A" BORDER="0"> resolution, revealing its close similarity to actin and MreB. The crystals contain AMPPNP-bound monomeric MamK in two different conformations. To investigate conformational changes associated with polymerization, we used unmodified MamK protein and cryo-EM with helical 3D reconstruction in RELION to obtain a density map and a fully refined atomic model of MamK in filamentous form at 3.6-[IMG]f1.gif" ALT="A" BORDER="0"> resolution. The filament is parallel (polar) double-helical, with a rise of 52.2 [IMG]f1.gif" ALT="A" BORDER="0"> and a twist of 23.8{degrees}. As shown previously and unusually for actin-like filaments, the MamK subunits from each of the two strands are juxtaposed, creating an additional twofold axis along the filament. Compared with monomeric MamK, ADP-bound MamK in the filament undergoes a conformational change, rotating domains I and II against each other to further close the interdomain cleft between subdomains IB and IIB. The domain movement causes several loops to close around the nucleotide-binding pocket. Glu-143, a key residue for catalysis coordinating the magnesium ion, moves closer, presumably switching nucleotide hydrolysis upon polymerization--one of the hallmarks of cytomotive filaments of the actin type.
