FIG. 9.

Speculative scaffold model for Bub1p. (A) The Bub1p-Bub3p complex is targeted to kinetochores. Proximity between proteins was used here to reflect dependencies for recruitment. For example, Mad2p requires Mad1p, and Bub3p and Bub1p are interdependent. (B) In wild-type cells, the N terminus of Bub1p then acts as a scaffold for the recruitment of further molecules of Bub3p, Mad3p, and Bub1p. Such a scaffolding role is supported by FRAP studies in vertebrates (26, 54), which have shown that Bub1p is a relatively stable component of outer kinetochores. (C) The Bub1Δ28-160 protein is unable to carry out this scaffolding role, and there is no further enrichment of Bub1p, Bub3p, or Mad3p at kinetochores in this mutant.