Prokaryotic origin of the actin cytoskeleton.
van den Ent F, Amos LA, Lowe J.
MRC Laboratory of Molecular Biology, Cambridge, UK. fent@mrc-lmb.cam.ac.uk
It was thought until recently that bacteria lack the actin or tubulin filament networks that organize eukaryotic cytoplasm. However, we show here that the bacterial MreB protein assembles into filaments with a subunit repeat similar to that of F-actin-the physiological polymer of eukaryotic actin.
By elucidating the MreB crystal structure we demonstrate that MreB and actin are very similar in three dimensions. Moreover, the crystals contain protofilaments, allowing visualization of actin-like strands at atomic resolution. The structure of the MreB protofilament is in remarkably good agreement with the model for F-actin, showing that the proteins assemble in identical orientations. The actin-like properties of MreB explain the finding that MreB forms large fibrous spirals under the cell membrane of rod-shaped cells, where they are involved in cell-shape determination. Thus, prokaryotes are now known to possess homologues both of tubulin, namely FtsZ, and of actin.
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EMBO J 2002 Dec 16;21(24):6935-43
F-actin-like filaments formed by plasmid segregation protein ParM.
van den Ent F, Moller-Jensen J, Amos LA, Gerdes K, Lowe J.
MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.
It was the general belief that DNA partitioning in prokaryotes is independent of a cytoskeletal structure, which in eukaryotic cells is indispensable for DNA segregation. Recently, however, immunofluorescence microscopy revealed highly dynamic, filamentous structures along the longitudinal axis of Escherichia coli formed by ParM, a plasmid-encoded protein required for accurate segregation of low-copy-number plasmid R1. We show here that ParM polymerizes into double helical protofilaments with a longitudinal repeat similar to filamentous actin (F-actin) and MreB filaments that maintain the cell shape of non-spherical bacteria. The crystal structure of ParM with and without ADP demonstrates that it is a member of the actin family of proteins and shows a domain movement of 25 degrees upon nucleotide binding. Furthermore, the crystal structure of ParM reveals major differences in the protofilament interface compared with F-actin, despite the similar arrangement of the subunits within the filaments. THUS, THERE IS NOW EVIDENCE FOR CYTOSKELETAL STRUCTURES, FORMED BY ACTIN-LIKE FILAMENTS THAT ARE INVOLVED IN PLASMID PARTITIONING IN E.COLI.
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