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J. Cell Biol.,
Volume 145, Number 5, May 31, 1999 933-950
Massachusetts Institute of Technology, Department of Biology, Cambridge, Massachusetts 02139
Kinetochores are smaller and more accessible experimentally in budding yeast than in any other
eukaryote. Believing that simple and complex kinetochores have important structural and functional properties in common, we characterized the structure of
CBF3, the essential centromere-binding complex that
initiates kinetochore formation in Saccharomyces cerevisiae. We find that the four subunits of CBF3 are multimeric in solution: p23Skp1 and p58Ctf13 form a heterodimer, and p64Cep3 and p110Ndc10 form homodimers.
Subcomplexes involving p58 and each of the other CBF3 subunits can assemble in the absence of centromeric DNA. In these subcomplexes, p58 appears to
function as a structural core mediating stable interactions among other CBF3 proteins. p58 has a short half-life in yeast, being subject to ubiquitin-dependent proteolysis, but we find that it is much more stable
following association with p64. We propose that
p23Skp1-p58-p64 complexes constitute the primary pool
of active p58 in yeast cells. These complexes can either
dissociate, reexposing p58 to the degradation pathway,
or can bind to p110 and centromeric DNA, forming a
functional CBF3 complex in which p58 is fully protected from degradation. This pathway may constitute
an editing mechanism preventing the formation of ectopic kinetochores and ensuring the fidelity of chromosome segregation.
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