Tubulins play crucial roles in cell division, intracellular traffic, and cell shape. Tubulin concentration is autoregulated by feedback control of messenger RNA (mRNA) degradation via an unknown mechanism. We identified tetratricopeptide protein 5 (TTC5) as a tubulin-specific ribosome-associating factor that triggers cotranslational degradation of tubulin mRNAs in response to excess soluble tubulin. Structural analysis revealed that TTC5 binds near the ribosome exit tunnel and engages the amino terminus of nascent tubulins. TTC5 mutants incapable of ribosome or nascent tubulin interaction abolished tubulin autoregulation and showed chromosome segregation defects during mitosis. Our findings show how a subset of mRNAs can be targeted for coordinated degradation by a specificity factor that recognizes the nascent polypeptides they encode.

Kinetochores are the central force-generating machines that move chromosomes during cell division. It is generally assumed that kinetochores move in an autonomous manner. However, we reveal here that movements of neighboring sister-kinetochore pairs in metaphase are correlated in a distance-dependent manner. This correlation increases in the absence of kinetochore oscillations or stable end-on attachments. This suggests that periodic movements of bioriented chromosomes limit the correlated motion of nonsisters. Computer simulations show that these correlated movements can occur when elastic crosslinks are placed between the K-fibers of oscillating kinetochores. Strikingly, inhibition of the microtubule crosslinking motor kinesin-5 Eg5 leads to an increase in nonsister correlation and impairs periodic oscillations. These phenotypes are partially rescued by codepletion of the kinesin-12 Kif15, demonstrating a function for kinesin-5 and kinesin-12 motors in driving chromosome movements, possibly as part of a crosslinking structure that correlates the movements of nonsister kinetochores.

The Ska complex is crucial for kinetochore–microtubule stability and is a substrate of Aurora B, master regulator of kinetochore–microtubule attachment dynamics. Here, Redli et al. show that Ska promotes Aurora B activity to limit its own microtubule and kinetochore association and ensure proper chromosome biorientation and segregation.

Chromosome alignment in the middle of the bipolar spindle is a hallmark of metazoan cell divisions. When we offset the metaphase plate position by creating an asymmetric centriole distribution on each pole, we find that metaphase plates relocate to the middle of the spindle before anaphase. The spindle assembly checkpoint enables this centering mechanism by providing cells enough time to correct metaphase plate position. The checkpoint responds to unstable kinetochore–microtubule attachments resulting from an imbalance in microtubule stability between the two half-spindles in cells with an asymmetric centriole distribution. Inactivation of the checkpoint prior to metaphase plate centering leads to asymmetric cell divisions and daughter cells of unequal size; in contrast, if the checkpoint is inactivated after the metaphase plate has centered its position, symmetric cell divisions ensue. This indicates that the equatorial position of the metaphase plate is essential for symmetric cell divisions.DOI: http://dx.doi.org/10.7554/eLife.05124.001

No abstract

The poles of the mitotic spindle contain one old and one young centrosome. In asymmetric stem cell divisions, the age of centrosomes affects their behaviour and their probability to remain in the stem cell. In contrast, in symmetric divisions, old and young centrosomes are thought to behave equally. This hypothesis is, however, untested. In this study, we show in symmetrically dividing human cells that kinetochore–microtubules associated to old centrosomes are more stable than those associated to young centrosomes, and that this difference favours the accumulation of premature end-on attachments that delay the alignment of polar chromosomes at old centrosomes. This differential microtubule stability depends on cenexin, a protein enriched on old centrosomes. It persists throughout mitosis, biasing chromosome segregation in anaphase by causing daughter cells with old centrosomes to retain non-disjoint chromosomes 85% of the time. We conclude that centrosome age imposes via cenexin a functional asymmetry on all mitotic spindles.DOI: http://dx.doi.org/10.7554/eLife.07909.001

The localization, mass, and dynamics of microtubules are important in many processes. Cells may actively monitor the state of their microtubules and respond to perturbation, but how this occurs outside mitosis is poorly understood. We used gene-expression analysis in quiescent cells to analyze responses to subtle and strong perturbation of microtubules. Genes encoding α-, β, and γ-tubulins (TUBAs, TUBBs, and TUBGs), but not δ- or ε-tubulins (TUBDs or TUBEs), exhibited the strongest differential expression response to microtubule-stabilizing versus destabilizing drugs. Quantitative PCR of exon versus intron sequences confirmed that these changes were caused by regulation of tubulin mRNA stability and not transcription. Using tubulin mRNA stability as a signature to query the Gene Expression Omnibus (GEO) database, we find that tubulin genes respond to toxins known to damage microtubules. Importantly, we find many other experimental perturbations, including multiple signaling and metabolic inputs that trigger tubulin differential expression, suggesting their novel, to our knowledge, role in the regulation of the microtubule cytoskeleton. Mechanistic follow-up confirms that one important physiological signal, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) activity, indeed regulates tubulin mRNA stability via changes in microtubule dynamics. We propose that tubulin gene expression is regulated as part of many coordinated biological responses, with wide implications in physiology and toxicology. Furthermore, we present a new way to discover microtubule regulation using transcriptomics.

Microtubules play crucial roles in cellular architecture, intracellular transport, and mitosis. The availability of free tubulin subunits impacts polymerization dynamics and microtubule function. When cells sense excess free tubulin, they trigger degradation of the encoding mRNAs, which requires recognition of the nascent polypeptide by the tubulin-specific ribosome-binding factor TTC5. How TTC5 initiates decay of tubulin mRNAs is unknown. Here, our biochemical and structural analysis reveals that TTC5 recruits the poorly studied protein SCAPER to the ribosome. SCAPER in turn engages the CCR4-NOT deadenylase complex through its CNOT11 subunit to trigger tubulin mRNA decay. SCAPER mutants that cause intellectual disability and retinitis pigmentosa in humans are impaired in CCR4-NOT recruitment, tubulin mRNA degradation, and microtubule-dependent chromosome segregation. Our findings demonstrate how recognition of a nascent polypeptide on the ribosome is physically linked to mRNA decay factors via a relay of protein-protein interactions, providing a paradigm for specificity in cytoplasmic gene regulation.