Interplay between Two Bacterial Actin Homologs, MamK and MamK-Like, Is Required for the Alignment of Magnetosome Organelles in Magnetospirillum magneticum AMB-1

Abstract

dMany bacterial species contain multiple actin-like proteins tasked with the execution of crucial cell biological functions. MamK, an actin-like protein found in magnetotactic bacteria, is important in organizing magnetosome organelles into chains that are used for navigation along geomagnetic fields. MamK and numerous other magnetosome formation factors are encoded by a genetic island termed the magnetosome island. Unlike most magnetotactic bacteria, Magnetospirillum magneticum AMB-1 (AMB-1) contains a second island of magnetosome-related genes that was named the magnetosome islet. A homologous copy of mamK, mamK-like, resides within this islet and encodes a protein capable of filament formation in vitro. Previous work had shown that mamK-like is expressed in vivo, but its function, if any, had remained unknown. Though MamK-like is highly similar to MamK, it contains a mutation that in MamK and other actins blocks ATPase activity in vitro and filament dynamics in vivo. Here, using genetic analysis, we demonstrate that mamK-like has an in vivo role in assisting organelle alignment. In addition, MamK-like forms filaments in vivo in a manner that is dependent on the presence of MamK and the two proteins interact in a yeast two-hybrid assay. Surprisingly, despite the ATPase active-site mutation, MamK-like is capable of ATP hydrolysis in vitro and promotes MamK filament turnover in vivo. Taken together, these experiments suggest that direct interactions between MamK and MamK-like contribute to magnetosome alignment in AMB-1.

Structure of the magnetosome‐associated actin‐like MamK filament at subnanometer resolution

“…22 A second MamK homolog, MamKlike, was identified in the AMB-1 genome, away from the magnetosome gene island. MamK-like was shown to co-localize with the MamK filament, 23,24 although the physiological relevance of this protein is poorly understood.…”

From invagination to navigation: The story of magnetosome-associated proteins in magnetotactic bacteria

“…MamK-like is a protein encoded outside of the MAI region in AMB-1 that is very similar to MamK, 65 but MamK was found to have a more dominant function in magnetosome chain formation. 99 In MSR-1 DmamK cells, magnetosome chains were detected, but were smaller, misplaced, less organized and with less ability to self-orient. 66 MamK was suggested to serve as a track that magnetosomes can move on 96,98 and to have a role in the assembly and localization of mature magnetosomes into a chain in the midcell for some strains, 58,62,66,96 which was further supported by in silico analysis of magnetosome formation, assembly, and localization.…”

“…In AMB‐1 ΔmamK cells, a MamK‐like protein creates filaments and magnetosomes are aligned into a chain, despite the lack of MamK (though not supported by another study). MamK‐like is a protein encoded outside of the MAI region in AMB‐1 that is very similar to MamK, but MamK was found to have a more dominant function in magnetosome chain formation . In MSR‐1 ΔmamK cells, magnetosome chains were detected, but were smaller, misplaced, less organized and with less ability to self‐orient .…”

Efficient genome editing ofMagnetospirillum magneticumAMB-1 by CRISPR-Cas9 system for analyzing magnetotactic behavior

“…The procedure for Western blotting was based on a general protocol as previously described [47]. Pelleted cells were resuspended in 20 mM Tris-HCl buffer (pH 7.4) and 5× protein-dyed buffer.…”