KINETOCHORES DISTINGUISH GTP FROM GDP FORMS OF THE MICROTUBULE LATTICE

During prometaphase in mitotic cell division, chromosomes attach to th e walls of microtubules and subsequently move to microtubule ends, whe re they stay throughout mitosis(1,2). This end-attachment seems to be essential for correct chromosome segregating. However, the mechanism b y which kinetochores, the multiprotein complexes that link chromosomes to the microtubules of the mitotic spindle(3,4), recognize and stay a ttached to microtubule ends is not understood. One due comes from the hydrolysis of GTP that occurs during microtubule polymerization. Altho ugh tubulin dimers must contain GTP to polymerize, this GTP is rapidly hydrolysed following the addition of dimers to a growing polymer. Thi s creates a microtubule consisting largely of GDP-tubulin, with a smal l cap of GTP-tubulin at the ends. It is possible that kinetochores dis tinguish the different structural states of a GTP-versus a GDP-microtu bule lattice, We have examined this question in vitro using reconstitu ted kinetochores from the yeast Saccharomyces cerevisiae. We found tha t kinetochores in vitro bind preferentially to GTP-rather than GDP-mic rotubules, and to the plus-end preferentially over the lattice. Our re sults could explain how kinetochores stay at microtubule ends and thus segregate chromosomes correctly during mitosis in vivo. This result d emonstrates that proteins exist that can distinguish the GTP conformat ion of the microtubule lattice.