Actin's origins

Anewly determined structure of a filament-forming protein from bacteria is eerily similar to that of eukaryotic actin.The bacterial protein, MreB, was known to affect cell shape in bacteria; Escherichia coli that lack MreB are spherical rather than rod-shaped. In March of this year, Jeffery Errington and colleagues (University of Oxford, UK) reported that filamentous helical structures containing MreB form close to the cell surface.

Fusinita van den Ent and colleagues (MRC Laboratory of Molecular Biology, Cambridge, UK) leapt on this finding. Beginning in March, they showed that purified MreB protein could form filaments. They then quickly crystallized and then solved the structure of MreB. They find that MreB and actin share all major structural elements, with just one small loop inserted into one of the helices of MreB, and they have very similar active sites.

Fortuitously, the MreB protein crystallized in what appear to be linear protofilaments, giving us the first detailed view of an actin-like interface (the interface for actin has been deduced only by modeling). The repeat unit in the crystals is 51 Å, comparable to the 55 Å spacing between polymerized actin subunits.

It is not known whether structural homology between actin and MreB will be reflected in a similar mechanism of action. To start addressing this question, van den Ent plans to sees whether the MreB filaments show any dynamic behavior. ▪

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