Kinetochore-microtubule interactions

Kinetochores are the multiprotein macromolecular assemblies on chromatin that ensure the accurate and timely segregation of chromosomes at during mitosis. To achieve this, kinetochores must interact with the microtubules of the spindle and microtubule-associated proteins. The nature of the kinetochore-microtubule interaction varies during the stages of the mitotic cycle, starting with initial capture and progressing through bi-orientation and congression at prometaphase/ metaphase, then finally separation of sister kinetochores/chromatids during anaphase. All the while during this process, kinetochores are able to signal their state of microtubule binding to the cell cycle control machinery. They are also able to influence microtubule dynamics in order to achieve chromosome segregation. Determining the structure and biochemistry of these various interactions continues to be a major objective of research in this field. Much of the cell biology/cytology of cell division has originally been described in metazoan cells in culture, e.g. PtKcells, newt lung, mouse and human cell lines etc., as their relatively large size and flat growth characteristics in culture make them easy to work with. However recent advances in microscopy, particularly fluorescence techniques, have made it possible to visualise spindle components in living cells of both budding and fission yeast, and also Drosophila cells, all of which had previously proven too small to image successfully, but which have powerful advantages in terms of genetics and proteomics. A great deal of research on kinetochore structure and function has been performed using the budding yeast Saccharomyces cerevisiae as an experimental system. Many of the proteins that make up the yeast kinetochore have counterparts in other organisms, including mammals (McAinsh et al. 2003). In addition, the yeast centromere DNA spans only about 130 bp (Hegemann and Fleig 1993), and a kinetochore interacts with a single microtubule in metaphase, compared with dozens in higher organisms (Winey et al. 1995; OToole et al. 1999). These features, coupled with the ease of genetic manipulation and the extensive proteomic information available have meant that much progress has been made in studying the yeast kinetochore. This chapter will examine, and compare where possible, the various kinds of interactions that occur between kinetochores and microtubules at different times during mitosis, both in yeast and in higher organisms.