Kinetochore-microtubule interactions: the means to the end - PubMed (original) (raw)

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Kinetochore-microtubule interactions: the means to the end

Tomoyuki U Tanaka et al. Curr Opin Cell Biol. 2008 Feb.

Abstract

Kinetochores are proteinaceous complexes containing dozens of components; they are assembled at centromeric DNA regions and provide the major microtubule attachment site on chromosomes during cell division. Recent studies have defined the kinetochore components comprising the direct interface with spindle microtubules, primarily through structural and functional analysis of the Ndc80 and Dam1 complexes. These studies have facilitated our understanding of how kinetochores remain attached to the end of dynamic microtubules and how proper orientation of a kinetochore-microtubule attachment is promoted on the mitotic spindle. In this article, we review these recent studies and summarize their key findings.

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Figures

Figure 1

Figure 1. Overview of kinetochore-microtubule interactions

The figure depicts kinetochore-microtubule interactions during prometaphase (steps 1–3), metaphase (step 4) and anaphase A (step 5). 1) Kinetochores are initially captured by the lateral surface of single microtubules that extend from one of the spindle poles [–8]. The initial kinetochore encounter with microtubules happens quickly following nuclear envelope breakdown (metazoan cells) [6,7] or once kinetochore assembly is complete (budding yeast: note that spindle poles of this organism have not yet separated in step 1–2) [53]. 2) Once captured, kinetochores are transported along the lateral surface of single microtubules toward the spindle pole (sliding) [–8]. Subsequently, at least in budding yeast, kinetochores are tethered at the end of the single microtubules and transported further as the microtubules shrink (end-coupled pulling) [40,53]. 3) As kinetochores approach spindle poles, both sister kinetochores attach to microtubules. If both kinetochores attach to microtubules from the same spindle pole, kinetochore-spindle pole connections by microtubules are re-oriented until proper bi-orientation is established [5,9]. 4) Cessation of re-orientation is dependent on the tension that is generated by microtubules upon establishment of bi-orientation [5,9]. The number of microtubules whose plus ends attach to a single kinetochore increases when tension is applied in metazoan cells [107], while only a single microtubule is thought to attach to a each kinetochore in budding yeast [62] (the latter case is shown here for simplicity). 5) Once all kinetochores bi-orient on the spindle, cohesion between sister chromatids is removed, causing sister chromatid segregation to opposite spindle poles during anaphase A [11]. Kinetochores are end-coupled and pulled poleward as the microtubules depolymerize [12,13].

Figure 2

Figure 2. The Ndc80 csomplex and the KMN network

A) Schematic of the 4-subunit Ndc80 complex indicating the constituent parts and defined domains. Panels on the right show the rod-like structure of the complex visualized by electron microscopy of individual rotary-shadowed recombinant complexes (scale bar 100 nm; reprinted from ref. [23]). The ribbon diagrams below show the calponin-homology domain of the Ndc80 subunit (residues 81–196 of human Ndc80, also known as Hec1) and comparison with the EB1 CH domain (reprinted from [31]). B) Microtubule-binding of the Ndc80 complex. The Ndc80 complex binds to the microtubule lattice with a fixed orientation forming “barbs” that extend away from the lattice. The microtubule-binding activity is located in the globular region of the Ndc80/Nuf2 dimer and is severely reduced by removal of the N-terminal extension on Ndc80. The “barbs” have a uniform polarity and binding angle indicating a specific binding site on the lattice (scale bar 200 nm; reprinted from [30]). C) Schematic of the KNL-1/Mis12 complex/Ndc80 complex (KMN) complex network. Direct association of the Ndc80 complex with these other two kinetochore constituents is conserved in fungi, nematodes, insects, and vertebrates [3]. The Spc24/25 dimer is required for association with KNL-1 and the Mis12 complex. KNL-1 also directly binds to microtubules. Aurora B negatively regulates the microtubule-binding activity of the Ndc80 complex by phosphorylating the basic N-terminal extension of the Ndc80 subunit [30].

Figure 3

Figure 3. Dam1 complexes oligomerize to form a ring encircling a microtubule

A) Electron micrographs of negatively stained microtubules in the presence of the Dam1 complexes. Scale bar, 50 nm. Reprinted from [44]. B) If the Dam1 complexes formed rings around microtubules, they would be accumulated at the microtubule plus end during the outward curling of protofilaments that accompanies depolymerization; this accumulation was observed in vitro [46] and in vivo [40]. C) The Dam1 complex has a crucial role in tethering kinetochores at microtubule ends and in converting microtubule depolymerization into kinetochore pulling force [40,46,49]. D) Electron micrographs of rings (negative stain and 16-fold rotational average), formed by the Dam1 complexes containing either a wild-type Dam1 protein (left) or a C-terminus (140 residues)-deleted Dam1 protein. A single Dam1 complex is shown by the dotted line. Scale bars, 25 nm. Reprinted from [73].

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