Beware the Borg

Too 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.Ian Macara (University of Virginia, Charlottesville, VA) 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 yeast genetics and septin antibody experiments in mammalian cells. Septins in cytokinesis may form a docking site for addition of new membrane. The details of this process, or why it requires a protein that can form filaments, remain a mystery. ▪

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