Mechanism of kinetochore assembly in maize
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The centromere is an essential chromatin structure for cell division and involves centromeric DNAs and their associated kinetochore. The kinetochore is a large complex of proteins that links centromere to spindle microtubules. Although studies of plant centromere/kinetochore have been especially active in recent years, few kinetochore proteins have been well characterized and the mechanism of kinetochore assembly remains unclear. Here I present cytological, biochemical, and transgenic data that contribute to our understanding of plant kinetochores. NDC80 is important for formation of the kinetochore-microtubule interface in yeast and animals. Although NDC80 sequences were found in plants, their function as a kinetochore protein had not been established. Using antibodies specific to maize NDC80, the protein was localized at centromeres and outside of the inner kinetochore protein CENP-C (CENPC), supporting the general view of NDC80 as a central kinetochore protein. NDC80 is constitutively present on maize kinetochores, which is different from non-plants where the association of NDC80 varies with the cell cycle. The continuous presence of NDC80 at centromeres suggests it has a important role in the kinetochore. With the complexity and variety of centromeric DNA, it is a major challenge to illustrate how kinetochores recognize centromeres. CENPC is a good candidate to investigate the interaction among the foundation proteins that are constitutively and closely associated with centromeres. By gel mobility shift assays, maize CENPC was shown to have DNA and RNA binding capacity. The major binding domain maps to a duplicated exon region. In vivo transgenics demonstrated that the binding domain is required for the centromere localization of CENPC. Based on a novel observation that RNA facilitates the CENPC-DNA interaction, we propose a mechanism for the involvement of centromere RNA in kinetochore assembly.