Molecular organization of kinetochores

Kinetochores link chromosomal centromeres to microtubules of the mitotic spindle. The kinetochores of budding yeast are assemblies of over 50 distinct protein species, nearly all of which have homologs in other eukaryotes. The budding yeast kinetochore appears to represent a single module of a more complex, parallel-module organization in most other species. The kinetochore components come together into well-defined protein complexes, which have an elaborate network of regulated contacts with each other, with centromeric chromatin, and with microtubules. We have undertaken to build up a three-dimensional picture of a yeast kinetochore, starting with efforts to crystallize some of its constituent protein complexes.  Conceptually, one can begin at either end -- the centromeric chromatin or the microtubule.  Ndc80, a 600 Å-long, rod-like heterotetramer, is the principal microtubule contact; a specialized nucleosome marks the chromatin end; a set of additional protein complexes connect the two.  Our on-going studies focus on these connector complexes and on how they recruit further components of the mitotic machinery.