Beware the Borg

newly 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 A Actin (left) and its bacterial cousin, MreB (right). 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. ᭿ Beware the Borg oo much of a Borg can be a bad thing. Just how and why it is bad may give clues to the function of septin proteins. and colleagues have found that certain mammalian Borg proteins, first characterized as Cdc42 effectors, can also bind to septins. An excess of the Borgs induces the formation of long and thick septin filaments, and expression of the septin-binding domain of a Borg protein clusters all visible septins into one perinuclear spot in the cell. The Rho GTPase Cdc42, which has been implicated previously in cytokinesis, vesicular transport, and cell polarity, inhibits septin binding to Borgs. Macara suspects that Cdc42 may inhibit another, unknown factor that normally unfolds Borg proteins so that their septin-binding domains are exposed. The complete disruption of septin organization by a Borg protein fragment might give clues to normal septin function. As yet, the only cellular disruption that Macara has noted is some problems with cytokinesis, which would be consistent with prior …