#kinetochore

Cell Cycle Regulation

What is the correct type of attachment between chromosomes and microtubules?

Kinetochore-microtubule interactions evolve through search and capture mechanism mediated by dynamic microtubule instability. Successful biorientation defined as amphitelic configuration where sister kinetochores attached to microtubules originating from opposite spindle poles. This arrangement produces pulling forces toward poles resisted by centromeric cohesin complex linking sisters, generating inter-kinetochore tension about 0.5 to 1 micron stretch detectable as separation between CENP-A foci. Tension acts as stabilization signal and silences spindle checkpoint. By contrast monotelic state

Ref: Cimini, Merotelic Attachment and Chromosome Missegregation, Curr Biol 2008; Alberts et al., Molecular Biology of the Cell, Chapter 17, Chromosome Alignment.

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Cell Cycle Regulation

How does Aurora B kinase correct incorrect kinetochore attachments?

Early prometaphase generates many attachment errors because kinetochores initially capture microtubules stochastically. Syntelic where both sisters attach same pole, monotelic where only one sister attached, and merotelic where one kinetochore attached to both poles often arise. These erroneous configurations lack balanced tension between sister centromeres. Aurora B kinase, as catalytic component of chromosomal passenger complex concentrated at inner centromere between sisters, senses low tension via proximity gradient. Active when kinetochores are close, Aurora B phosphorylates N-terminal ta

Ref: Lampson & Cheeseman, Aurora B-Dependent Error Correction of Kinetochore Attachments, Nat Rev Mol Cell Biol 2011; Nature Communications, CPC.

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Cell Cycle Regulation

Which protein ensures correct kinetochore-microtubule attachment?

Attachment of spindle microtubules to chromosomes occurs via kinetochore, megadalton protein assembly built on centromeric CENP-A chromatin. Outer kinetochore KMN network provides primary microtubule interface. Ndc80 complex heterotetramer Ndc80, Nuf2, Spc24, Spc25 extends 55 nm, with calponin homology domains at Ndc80-Nuf2 binding microtubule lattice electrostatically, while Spc24-Spc25 anchors to inner kinetochore Mis12 complex. Affinity modulated by Aurora B phosphorylation at Ndc80 tail reducing binding upon low tension to facilitate error correction. In budding yeast, Dam1 complex decamer

Ref: Cheeseman & Desai, Molecular Architecture of KMN Network, Annu Rev Cell Dev Biol 2008; Joglekar & DeLuca, Kinetochore-Microtubule Interface.

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Cell Cycle Regulation

Which kinase phosphorylates Knl1 to recruit checkpoint proteins at unattached kinetochores?

Assembly of spindle assembly checkpoint proteins at kinetochores requires phosphorylation-dependent creation of docking sites on outer kinetochore scaffold. KNL1, also called Blinkin or Spc105, contains multiple MELT repeats (Met-Glu-Leu-Thr) that become phosphorylated when kinetochore is unattached. The kinase responsible is monopolar spindle 1, abbreviated Mps1, conserved from yeast to humans, dual-specificity kinase that localizes to unattached kinetochores via its TPR domain interacting with Ndc80 complex and calponin homology domains. Mps1 phosphorylation of MELT motifs generates binding

Ref: London & Biggins, Genes Dev 2014, Mps1 and KNL1 Phosphorylation; Vleugel et al., J Cell Biol 2015, MELT Repeats.

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Nucleosome Packing

CENP-A mainly localizes to:

CENP-A containing nucleosomes localize almost exclusively to functional centromere, not dispersed throughout chromosome arms. High resolution immunofluorescence and chromatin immunoprecipitation sequencing show discrete foci at primary constriction colocalizing with inner kinetochore markers CENP-C and CENP-T during interphase and mitosis. At centromere CENP-A directly recruits CCAN network initiating kinetochore assembly ensuring spindle microtubule attachment and checkpoint signaling. It does not accumulate at telomeres containing TTAGGG repeats protected by shelterin, nor at nucleolar organ

Ref: Earnshaw and Migeon CENP-A Localization; Lodish et al., Chapter 19: CENP-A Mainly Localizes to Centromere

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Eukaryotic chromatin and chromosome

Kinetochore assembles at:

Kinetochore is large multiprotein structure assembling exclusively at centromeric chromatin containing CENP-A nucleosomes. During mitosis inner kinetochore components CENP-C, CENP-T, CENP-I constitutively associate with centromere, outer KMN network including Ndc80, Mis12, KNL1 complexes binds plus ends of spindle microtubules. This linkage transmits pulling forces to separate sister chromatids and activates spindle assembly checkpoint via Mad2, BubR1 until biorientation achieved. Telomeres cap ends, replication origins fire throughout genome, NORs build nucleolus, whereas kinetochore location

Ref: Cheeseman and Desai 2008 Nat Rev Mol Cell Biol; Lodish et al., Chapter 19: Kinetochore Assembles at Centromere

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Eukaryotic chromatin and chromosome

Centromere function is:

Centromeres provide mechanical linkage essential for chromosome segregation. At centromeric DNA, histone H3 variant CENP-A replaces canonical H3 assembling specialized nucleosomes that directly recruit constitutive centromere associated network CCAN proteins including CENP-C and CENP-T. This platform builds kinetochore, large multiprotein structure containing KMN network that captures spindle microtubule plus ends during mitosis and meiosis. It ensures biorientation, tension sensing via Aurora B kinase and equal segregation. Without functional centromere kinetochore cannot assemble leading to

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 19: Centromere Function - Spindle Attachment via Kinetochore

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