Regulatory mechanisms of kinetochore-microtubule interaction in mitosis

Abstract

Interaction of microtubules with kinetochores is fundamental to chromosome segregation. Kinetochores initially associate with lateral surfaces of microtubules and subsequently become attached to microtubule ends. During these interactions, kinetochores can move by sliding along microtubules or by moving together with depolymerizing microtubule ends. The interplay between kinetochores and microtubules leads to the establishment of bi-orientation, which is the attachment of sister kinetochores to microtubules from opposite spindle poles, and subsequent chromosome segregation. Molecular mechanisms underlying these processes have been intensively studied over the past 10 years. Emerging evidence suggests that the KNL1-Mis12-Ndc80 (KMN) network plays a central role in connecting kinetochores to microtubules, which is under fine regulation by a mitotic kinase, Aurora B. However, a growing number of additional molecules are being shown to be involved in the kinetochore-microtubule interaction. Here I overview the current range of regulatory mechanisms of the kinetochore-microtubule interaction, and discuss how these multiple molecules contribute cooperatively to allow faithful chromosome segregation.

Fig. 1

Process of kinetochore capture and transport during chromosome segregation. Shown are kinetochore–microtubule interactions in prophase (a), prometaphase (b and c), metaphase (d), and anaphase (e). For simplicity, only two pairs of sister chromatids are shown. Arrows indicate direction of kinetochore motion or force (tension) exerted on kinetochores. Kinetochores initially attach to the lateral surface of microtubules (lateral attachment; b), and slide along microtubules toward spindle poles (lateral sliding; b). Lateral attachment is then converted to end-on attachment (c), followed by K-fiber formation. Kinetochores attached to microtubules in an end-on manner are pulled toward spindles poles concomitantly with microtubule depolymerization (end-on pulling; c). When sister kinetochores attach to microtubules from opposite spindle poles (bi-orientation), chromosomes align on the metaphase plate (congression; d). Once all the chromosomes align on the metaphase plate, sister chromatids segregate toward respective spindle poles, pulled by shrinking K-fibers (e)

Fig. 2

Kinetochore structure of mammalian cells. a Structure of the spindle. b A schematic of kinetochore structure found by electron microscopy. The structure of the boxed area in a is shown. c A schematic of the molecular structure of the kinetochore. The structure of the boxed area in b is shown. CPC chromosome passenger complex

Fig. 3

Mechanisms facilitating kinetochore–microtubule attachment and spindle formation. a Search and capture: microtubules nucleated at centrosomes elongate in various directions. When microtubules do not encounter kinetochores, they eventually shrink. On the other hand, microtubules are stabilized when they attach to kinetochores, promoting overall kinetochore capture. Arrows represent microtubule growth and shrinkage. b RCC1 localization on chromatin promotes microtubule assembly near the chromosomes due to increased levels of RanGTP around chromosomes. The microtubules formed around chromosomes then promote spindle formation in the absence of centrosomes. Arrows indicate microtubule formation around chromosomes. c Microtubules can be generated not only at spindle poles, but also at kinetochores. Such kinetochore-derived microtubules cooperate with microtubules from spindle poles for efficient spindle formation. Arrows indicate microtubule generation at kinetochores

Fig. 4

Correct and incorrect attachments of kinetochores to microtubules. i Amphitelic (bipolar) attachment, which is correct. ii–iv Incorrect attachments; ii monotelic monopolar attachment, iii syntelic monopolar attachment, iv merotelic bipolar attachment

Fig. 5

Correction of erroneous kinetochore–microtubule attachments by Aurora B. A schematic diagram depicting the ‘spatial separation model’: a In the case of syntelic attachment, sister kinetochores are not stretched due to the lack of tension. The resulting proximity of the kinetochores to the CPC at inner centromere makes the KMN network highly phosphorylated (shown as ‘P’) by Aurora B, thus kinetochore–microtubule attachments are destabilized. b When kinetochores are bi-oriented, they are stretched both internally and externally due to the tension exerted between them. As a result, the KMN network is under-phosphorylated and then kinetochore–microtubule attachments are stabilized. When KNL1 is dephosphorylated, PP1 is recruited to KNL1, further shifting the balance to dephosphorylation of the KMN network. Enrichment of the CPC on misaligned chromosomes, schematically shown as increased size of the oval representing the CPC in a, is another layer of mechanism for the differential phosphorylation of kinetochore substrates to ensure efficient error correction. c In the case of merotelic attachments, the kinetochore is stretched toward both spindle poles. Therefore, a portion of the microtubule attachment sites on the kinetochore tends to be closer to inner centromere, where the attachment is destabilized as a result of the phosphorylation of kinetochore substrates by Aurora B

Fig. 6

Revised model for the process of bi-orientation establishment. After kinetochore capture at the lateral surface of microtubules (a), laterally attached kinetochores slide along microtubules toward the middle of the forming spindle (b). There, chromosomes form a circular belt (chromosome ring), excluded from the center of the spindle with their arms protruding away from the spindle (c). On the chromosome ring, microtubules from both spindle poles with similar density efficiently form amphitelic attachments on the laterally attached kinetochores (d). See Fig. 1 for comparison