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Rocking on Pins

LGN opens to link NuMA and Gi at the cortex. MACARA/ELSEVIER

Mitotic spindle positioning involves a conformational switch in LGN, the mammalian version of the invertebrate Pins protein, according to Quansheng Du and Ian Macara (University of Virginia, Charlottesville, VA). Interference with the switch induces a spindle rocking that represents the first genetically manipulable model for studying spindle positioning in vertebrates.

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 Gi 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 Gi on the membrane.

Overexpressed Gi 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."

Reference:

Du, Q., et al. 2004. Cell. 119:503–516.[CrossRef][Medline]

William A. Wells

wellsw{at}rockefeller.edu

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This Article Full Text (PDF, 656K) PPT slides of all figures Alert me when this article is cited Services Email this article Similar articles in this journal Alert me to new content in the JCB Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Wells, W. A. Search for Related Content PubMed Articles by Wells, W. A. Social Bookmarking                            What's this?