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Published 28 April 2003. doi:10.1083/jcb.200212110
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© The Rockefeller University Press, 0021-9525/2003/4/295 $5.00
The Journal of Cell Biology, Volume 161, Number 2, 295-307

Article

Sim4

: a novel fission yeast kinetochore protein required for centromeric silencing and chromosome segregation



Alison L. Pidoux1,2, William Richardson2 and Robin C. Allshire1,2

1 Wellcome Trust Centre for Cell Biology, Institute of Cell and Molecular Biology, University of Edinburgh, Edinburgh EH9 3JR, UK
2 Medical Research Council Human Genetics Unit, Western General Hospital, Edinburgh EH4 2XU, UK

Address correspondence to Robin C. Allshire, Wellcome Trust Centre for Cell Biology, Institute of Cell and Molecular Biology, 6.34 Swann Building, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JR, UK. Tel.: 44-131-650-7117. Fax: 44-131-650-7778. E-mail: robin.allshire{at}ed.ac.uk

Fission yeast centromeres are composed of two domains: the central core and the outer repeats. Although both regions are required for full centromere function, the central core has a distinct chromatin structure and is likely to underlie the kinetochore itself, as it is associated with centromere-specific proteins. Genes placed within either region are transcriptionally silenced, reflecting the formation of a functional kinetochore complex and flanking centromeric heterochromatin. Here, transcriptional silencing was exploited to identify components involved in central core silencing and kinetochore assembly or structure. The resulting sim (silencing in the middle of the centromere) mutants display severe chromosome segregation defects. sim2+ encodes a known kinetochore protein, the centromere-specific histone H3 variant Cnp1CENP-A. sim4+ encodes a novel essential coiled-coil protein, which is specifically associated with the central core region and is required for the unusual chromatin structure of this region. Sim4 coimmunoprecipitates with the central core component Mis6 and, like Mis6, affects Cnp1CENP-A association with the central domain. Functional Mis6 is required for Sim4 localization at the kinetochore. Our analyses illustrate the fundamental link between silencing, chromatin structure, and kinetochore function, and establish defective silencing as a powerful approach for identifying proteins required to build a functional kinetochore.

Key Words: kinetochore; chromatin; centromere; silencing; chromosome segregation

The online version of this article includes supplemental material.

* Abbreviations used in this paper: ChIP, chromatin immunoprecipitation; IP, immunoprecipitate; MNase, micrococcal nuclease; MT, microtubule; TBZ, thiabendazole.


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