The CCAN complex: Linking centromere specification to control of kinetochore–microtubule dynamics

Abstract

For over 70 years, chromosomes have been known to oscillate back-and-forth on the metaphase plate. These movements are directed by kinetochores, the microtubule-attachment complexes on centromeres that regulate the dynamics of bound spindle microtubules. Recent evidence shows that the CCAN (Constitutive Centromere Associated Network) kinetochore network, which directly binds centromeric nucleosomes, plays a crucial role in the control of kinetochore microtubule dynamics. Here we review how this 15-subunit protein network functions within the kinetochore machinery, how it may adapt dynamically both in time and in space to the functional requirements necessary for controlled and faithful chromosome movements during cell division, and how this conserved protein network may have evolved in organisms with different cell division machineries.

Introduction

Chromosome segregation acheives faithful partitioning of the duplicated genetic material into the two daughter cells during mitosis. One of the key requirements for successful chromosome segregation is the establishment of a tight metaphase plate, which forms as the chromosomes congress towards the equator of the mitotic spindle [1], [2]. Kinetochores attach chromosomes to microtubules and play the central role in chromosome congression by controlling the dynamics of bound microtubules [3], [4], [5], [6], [7]. In addition, plus-end directed microtubule-motor forces generated by kinesins also play a role in the initial anti-poleward transport of chromosomes to the spindle equator. Once the metaphase plate is established kinetochore–microtubule dynamics then play the dominant role in controlling chromosome movements [8]. In most eukaryotes, the kinetochore is not bound to a single microtubule, but rather to bundles of microtubules, termed k-fibers, that, depending on the organism, can consist of up to 25 individual microtubules. When the majority of microtubules within a k-fiber undergo catastrophe the k-fiber will shrink. During shrinkage, the kinetochore will remain coupled to the depolymerizing plus-ends, which imparts a pulling force on the chromosomes. If the majority of microtubules within a k-fiber switch to a polymerizing state then the coupled-kinetochore will exert a pushing force on the chromosome. While pushing forces can contribute in certain situations to this process, it is the pulling forces generated at the leading sister-kinetochore are the dominant drivers of poleward movement of the chromosome [9], [10], [11].

In the vast majority of metazoan cells, bipolar-attached chromosomes do not align onto the metaphase plate in a single movement, but rather oscillate back and forth along the spindle axis in regular movements that continue after a chromosome reaches the spindle equator [8], [12], [13], [14]. The regularity is highly damped and correlations between oscillations do not extend over more than a full period of 75 s. It is unknown whether this emergent oscillatory behavior is simply a reflection of the mechanochemical systems, which underpin directional switching, or whether this regularity is itself physiologically important [15]. In either case, the regularity points to a highly regulated and coordinated control system at the kinetochore, as opposed to purely stochastic microtubule-based dynamic instability [16]. At the molecular level the human CCAN (Constitutive Centromere Associated Network) kinetochore complex was recently identified as a key component of this machinery that directly controls the dynamics of kinetochore–microtubules [17].