"In test tube experiments, we've found that the kinetochore proteins form rings around the microtubules and this ring formation promotes microtubule assembly, stabilizes against disassembly, and promotes bundling," says Eva Nogales, a biophysicist who holds joint appointments with the Lawrence Berkeley National Laboratory (Berkeley Lab), the University of California at Berkeley, and the Howard Hughes Medical Institute (HHMI). "If ring formation takes place in vivo, it could be the mechanism by which chromosomes are kept segregated during mitosis."
Nogales is one of the co-authors of a paper reporting the results of this research which appears in the January 21, 2005 issue of the journal Molecular Cell. Other authors of the Molecular Cell paper were Georjana Barnes, David Drubin and Stefan Westermann, with UC Berkeley's Department of Molecular and Cell Biology, who were the lead investigators on this work, plus Agustin Avila-Sakar and Hong-Wei Wang, with Berkeley Lab, and Hanspeter Niederstrasser and Jonathan Wong with UC Berkeley.
Says Barnes, "Mistakes in chromosome segregation during mitosis contribute to cancer and birth defects. From various genetic experiments we know that the activity of a 10-protein complex of kinetochores, called Dam1, is responsible for the faithful segregation of chromosomes during mitosis. While we don't know at this time that ring formation occurs in vivo, we do see in our in vitro tests that Dam1 ring formation strengthens the microtubules."
Thousands of microtubule fibers are woven togethe