Abstract
It is now recognized that actin-like proteins are widespread in bacteria and, in contrast to eukaryotic actins, are highly diverse in sequence and function. The bacterial actin, MamK, represents a clade, primarily found in magnetotactic bacteria, that is involved in the proper organization of subcellular organelles, termed magnetosomes. We have previously shown that MamK from Magnetospirillum magneticum AMB-1 (AMB-1) forms dynamic filaments in vivo. To gain further insights into the molecular mechanisms that underlie MamK dynamics and function, we have now studied the in vitro properties of MamK. We demonstrate that MamK is an ATPase that, in the presence of ATP, assembles rapidly into filaments that disassemble once ATP is depleted. The mutation of a conserved active site residue (E143A) abolishes ATPase activity of MamK but not its ability to form filaments. Filament disassembly depends on both ATPase activity and potassium levels, the latter of which results in the organization of MamK filaments into bundles. These data are consistent with observations indicating that accessory factors are required to promote filament disassembly and for spatial organization of filaments in vivo. We also used cryo-electron microscopy to obtain a high resolution structure of MamK filaments. MamK adopts a two-stranded helical filament architecture, but unlike eukaryotic actin and other actin-like filaments, subunits in MamK strands are unstaggered giving rise to a unique filament architecture. Beyond extending our knowledge of the properties and function of MamK in magnetotactic bacteria, this study emphasizes the functional and structural diversity of bacterial actins in general.
Highlights
The bacterial actin MamK is involved in the organization of bacterial organelles called magnetosomes
We have previously shown that MamK from Magnetospirillum magneticum AMB-1 (AMB-1) forms dynamic filaments in vivo
We characterized the in vitro properties of the bacterial actin MamK from M. magneticum AMB-1, which is involved in the organization and positioning of organelles in magnetotactic bacteria
Summary
The bacterial actin MamK is involved in the organization of bacterial organelles called magnetosomes. Bacterial actins form filamentous structures, and the majority of studies far have concentrated on a few select groups, such as the functionally distinct MreB and ParM proteins [11, 12]. MamK proteins form their own phylogenetic group within the bacterial actins and have been implicated in the organization of subcellular organelles in magnetotactic bacteria [19, 20] This phylogenetically diverse group of bacteria can form intracellular structures called magnetosomes, which are inner membrane invaginations that direct the formation of magnetic crystals. These earlier studies presented valuable new insights; they provided only limited, and somewhat contradicting, information about assembly and filament dynamics and the role of nucleotides for MamK These studies employed Histagged derivatives of MamK that could exhibit different in vitro properties than the native untagged protein
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