Peter K. Sorger
An Analysis of Microtubule Binding to Simple and Complex KinetochoresThe orderly segregation of chromosomes at mitosis is essential for the faithful transmission of genetic information. We are studying kinetochores, the structures that anchor and move chromosomes along the microtubules of the mitotic spindle. Kinetochores are composed of centromeric DNA and a set of proteins whose identities and functions are largely unknown. The goal of our work is to identify and analyze the proteins in kinetochores that link centromeric DNA to microtubules and to thereby elucidate the mechanisms that regulate chromosome movement.
The approach we are taking is to determine the composition and biochemical properties of kinetochores in vitro and then, using genetics, to relate these properties to the functions of kinetochores in vivo. Our work has started with the simplest of all known kinetochores, those from S. cerevisiaie. We have developed methods to reconstitute budding yeast kinetochore complexes on exogenous centromeric DNA and, using fluorescence microscopy, to measure the binding of these complexes to microtubules. By analyzing microtubule attachment activity in extracts from mutant yeast strains, we have shown that attachment requires at least three of the four subunits of a protein complex known as CBF3. CBF3 binds to the essential core region of the yeast centromere. Genes encoding CBF3 components are required for chromosome segregation and for cell growth. Although CBF3 is nece