Chl4p and iml3p are two new members of the budding yeast outer kinetochore.

Kinetochore proteins contribute to the fidelity of chromosome transmission by mediating the attachment of a specialized chromosomal region, the centromere, to the mitotic spindle during mitosis. In budding yeast, a subset of kinetochore proteins, referred to as the outer kinetochore, provides a link between centromere DNA-binding proteins of the inner kinetochore and microtubule-binding proteins. Using a combination of chromatin immunoprecipitation, in vivo localization, and protein coimmunoprecipitation, we have established that yeast Chl4p and Iml3p are outer kinetochore proteins that localize to the kinetochore in a Ctf19p-dependent manner. Chl4p interacts with the outer kinetochore proteins Ctf19p and Ctf3p, and Iml3p interacts with Chl4p and Ctf19p. In addition, Chl4p is required for the Ctf19p-Ctf3p and Ctf19p-Iml3p interactions, indicating that Chl4p is an important structural component of the outer kinetochore. These physical interaction dependencies provide insights into the molecular architecture and centromere DNA loading requirements of the outer kinetochore complex.