The switch operates via the opening of LGN, which can normally fold back onto itself. The unfolding is triggered by binding to either membrane-bound Gαi or NuMA, a protein that localizes to and stabilizes spindle poles. Unfolding was detected both as lack of self-binding and a mitotic reduction in FRET between fluorophores placed on either end of LGN.
In the cell, binding of NuMA to LGN appears to be a prerequisite for further interactions, as in cells lacking NuMA the LGN never made it to the cortex. NuMA and its microtubule-bundling activity are nuclear during interphase. During mitosis, NuMA is liberated when the nuclear envelope is dissolved and can now pry apart cytoplasmic LGN. This makes the LGN–NuMA complex available for binding to Gαi on the membrane.
Overexpressed Gαi or LGN induced rocking of the spindle, perhaps thanks to unequal pulling forces on the spindle. Macara hopes to sort out the basis for this rocking by visualizing the behaviors of individual microtubules near the cortex. Meanwhile, the rocking should be invaluable as a readout of spindle–cortex attachment. “It's artificial,” says Macara, “but it gives us a really powerful tool to look at forces on astral microtubules during mitosis.”