Biology subjects

Sobajima, T.

Publications and source records attributed to Sobajima, T..

3 recordsLinked to original sources

bioRxiv2023

MDM2 acts as a timer reporting the length of mitosis

Delays in mitosis trigger p53-dependent arrest in G1 of the following cell cycle, enabling cells to respond to changes that would otherwise promote chromosome instability and aneuploidy 1-10. We find that MDM2, the p53 ubiquitin ligase, is a key component of the timer mechanism triggering G1 arrest in response to prolonged mitosis. This timer function arises because MDM2 has a short half-life and ongoing protein synthesis is therefore necessary to maintain its steady-state concentration. Due to the attenuation of protein synthesis in mitosis, the amount of MDM2 gradually falls during mitosis, but normally remains above a critical threshold for p53 regulation at the onset of G1. When mitosis is extended by prolonged spindle assembly checkpoint activation, the amount of MDM2 drops below this threshold, stabilising p53. Subsequent p53-dependent p21 accumulation in the following G1 then channels cells into a prolonged cell cycle arrest, whereas abrogation of the response in p53-deficient cells allows them to bypass this crucial defence mechanism.

cell biology↗

bioRxiv2022

PP6-regulated Aurora A activity towards NDC80 limits mitotic spindle size to prevent micronucleation

Amplification of the mitotic kinase Aurora A or loss of its regulator protein phosphatase 6 (PP6) have emerged as drivers of genome instability. Cells lacking PPP6C, the catalytic subunit of PP6, have amplified Aurora A activity and as we show here, enlarged mitotic spindles which fail to hold chromosomes tightly together in anaphase, causing defective nuclear structure. Using functional genomics to shed light on the processes underpinning these changes, we discover synthetic lethality between PPP6C and the kinetochore protein NDC80. We find that NDC80 is phosphorylated on multiple N-terminal sites during spindle formation by Aurora A-TPX2, exclusively at checkpoint-silenced, microtubule-attached kinetochores. NDC80 phosphorylation persists until spindle disassembly in telophase, is increased in PPP6C-knockout cells and, and is Aurora B-independent. An Aurora-phosphorylation-deficient NDC80-9A mutant reduces spindle size and suppresses defective nuclear structure in PPP6C-knockout cells. By regulating NDC80 phosphorylation by Aurora A-TPX2, PP6 plays an important role in mitotic spindle formation and size control, and thus the fidelity of cell division.

cell biology↗

bioRxiv2021

A baton-relay mechanism for ER retrieval signal capture and proofreading by the KDEL receptor

The KDEL-retrieval pathway captures escaped ER proteins with a KDEL or variant C-terminal signal at acidic pH in the Golgi and releases them at neutral pH in the ER. To address the mechanism of signal binding and the molecular basis for differences in signal affinity, we determined the HDEL and RDEL bound structures of the KDEL-receptor. Affinity differences are explained by interactions between the variable -4 position of the signal and W120, whereas initial capture of retrieval signals by their carboxyl-terminus is mediated by a baton-relay mechanism involving a series of conserved arginine residues in the receptor. This explains how the signal is first captured and then pulled into the binding cavity. During capture, retrieval signals undergo a selective proofreading step involving two gatekeeper residues D50 and E117 in the receptor. These mechanisms operate upstream of the pH-dependent closure of the receptor and explain the selectivity of the KDEL-retrieval pathway.

cell biology↗