Abstract
The budding yeast kinetochore is ~68 nm in length with a diameter slightly larger than a 25 nm microtubule. The kinetochores from the 16 chromosomes are organized in a stereotypic cluster encircling central spindle microtubules. Quantitative analysis of the inner kinetochore cluster (Cse4, COMA) reveals structural features not apparent in singly attached kinetochores. The cluster of Cse4-containing kinetochores is physically larger perpendicular to the spindle axis relative to the cluster of Ndc80 molecules. If there was a single Cse4 (molecule or nucleosome) at the kinetochore attached to each microtubule plus end, the cluster of Cse4 would appear geometrically identical to Ndc80. Thus, the structure of the inner kinetochore at the surface of the chromosomes remains unsolved. We have used point fluorescence microscopy and statistical probability maps to deduce the two-dimensional mean position of representative components of the yeast kinetochore relative to the mitotic spindle in metaphase. Comparison of the experimental images to three-dimensional architectures from convolution of mathematical models reveals a pool of Cse4 radially displaced from Cse4 at the kinetochore and kinetochore microtubule plus ends. The pool of displaced Cse4 can be experimentally depleted in mRNA processing pat1Δ or xrn1Δ mutants. The peripheral Cse4 molecules do not template outer kinetochore components. This study suggests an inner kinetochore plate at the centromere-microtubule interface in budding yeast and yields information on the number of Ndc80 molecules at the microtubule attachment site.