Kinetochores expand their reach

Kinetochores attach chromosomes to the mitotic spindle. After binding initially to the sides of spindle microtubules, they form stable attachments to microtubule plus ends so that chromosomes can be segregated to the spindle poles during anaphase. In addition , kinetochores recruit the spindle assembly checkpoint proteins that prevent cells from entering anaphase if their chromosomes aren't attached to the spindle correctly. Although the function of individual kinetochore proteins is fairly well understood , how their assembly and disassembly is orchestrated to coordinate the kineto-chore's different functions remains unclear. Wynne and Funabiki use super-resolution microscopy to reveal that, in the absence of microtubule attachment, a subset of kineto-chore proteins form extended fi laments that could help activate the spindle checkpoint and capture microtubules (1). Kinetochores are thought to assemble in a hierarchical order (2). Inner kinetochore proteins, such as CENP-C and CENP-T, fi rst bind to centromeric chroma-tin, which is marked by the variant histone CENP-A. These proteins then recruit outer kinetochore proteins involved in microtubule attachment and spindle checkpoint signaling. The inner and outer layers of kineto-chores can be distinguished by electron microscopy, but the organization of individual proteins within these layers remains uncertain. David Wynne and Hironori Fun-abiki, from The Rockefeller University in New York, decided to examine kinetochores using 3D structured-illumination microsco-py. " Not many studies have looked at kine-tochores using super-resolution microscopy, so we wanted to see what kinds of structures we could visualize, " Funabiki explains. The researchers looked at the assembly of kinetochores on sperm chromosomes added to Xenopus egg extracts. " The surprising thing was that several outer kinetochore proteins showed a massive expansion when the extracts were treated with nocodazole to depolymerize microtubules, " Funabiki says. These proteins—including the checkpoint proteins Bub1, BubR1, and Mad1, as well as proteins, such as CENP-E and dynein, involved in lateral microtubule attachment— formed long, thin fi laments that extended more than a micron away from centromeric chromatin marked by CENP-A. Wynne and Funabiki saw a similar, transient expansion of these proteins in unperturbed egg extracts at the beginning of mitosis. Outer kinetochore proteins have previously been seen to expand their distribution following microtu-bule depolymerization (3), but not to the extent observed in Xenopus egg extracts. Moreover, Wynne and Funabiki saw that the inner kinetochore protein CENP-C also localized to the expanded kinetochore fi laments. In contrast, the inner kineto-chore protein CENP-T and outer kineto-chore proteins involved …