Review series: The functions and consequences of force at kinetochores

Abstract

Chromosome segregation requires the generation of force at the kinetochore-the multiprotein structure that facilitates attachment of chromosomes to spindle microtubules. This force is required both to move chromosomes and to signal the formation of proper bioriented attachments. To understand the role of force in these processes, it is critical to define how force is generated at kinetochores, the contributions of this force to chromosome movement, and how the kinetochore is structured and organized to withstand and respond to force. Classical studies and recent work provide a framework to dissect the mechanisms, functions, and consequences of force at kinetochores.

Figure 1.

Simplified diagram of the kinetochore showing the major proteins involved in the DNA–microtubule attachment. (Left) The Ndc80 complex (dark blue) binds to microtubules and forms two separate connections to kinetochores. First, the Ndc80 complex binds to the Mis12 complex (green) and KNL-1 (magenta). The Mis12 complex in turn binds to CENP-C (orange), which binds to nucleosomes containing the histone H3 variant CENP-A (purple). Second, the Ndc80 complex binds to CENP-T (light blue). CENP-T interacts with DNA as a part of a heterotetrameric nucleosome-like CENP-T–W–S–X complex. In humans, the Ndc80 complex attachment to microtubules is enhanced by an interaction with the Ska1 complex (pink and blue; Schmidt et al., 2012). Additional components may form interactions between the two connective pathways (red). (Right) Upon microtubule depolymerization, the flexible protein components of the kinetochore may rearrange. For example, recent evidence has suggested that the N and C termini of CENP-T separate under tension (Suzuki et al., 2011) and that the subunits of the Mis12 complex redistribute (Wan et al., 2009).

Figure 2.

Models for force response at kinetochores at both the individual protein level and global scale. (A–C) We propose three nonexclusive models for how kinetochores respond to the application of force: kinetochore proteins with elastic properties could serve to absorb some of the force produced by depolymerizing microtubules (A), multiple weak interfaces could form parallel attachments between the depolymerizing microtubule and chromosome such that the force produced by the microtubule would be diffused across multiple connections (B), and additional kinetochore components could serve as dynamic cross-linkers to diffuse force and add interactions between pairs of proteins to strengthen the protein–protein interface (C). The kinetochore protein components themselves could have multiple responses at a molecular level including that (1) under pulling forces, the bonds holding together the tertiary and secondary structure of a protein can break, causing the protein to unfold. If reversible, this would provide elastic properties, but if permanent, could lead to loss of functional kinetochore components. (2) The force generated by kinetochores is directed toward the limited number of protein–DNA interactions formed between the kinetochore proteins and the chromosome. Some tension may be relieved as the DNA wrapped around adjacent nucleosomes is pulled. This first results in the straightening out of the compact “beads on a string” structure, but with sufficient pulling force, the nucleosomes would be removed from the DNA. (3) Protein–protein interfaces held together by noncovalent bonds can break under pulling force, but the presence of additional proteins to strengthen interactions could prevent the loss of important interfaces.