A high-sensitivity phospho-switch triggered by Cdk1 governs chromosome morphogenesis during cell division

Abstract

The initiation of chromosome morphogenesis marks the beginning of mitosis in all eukaryotic cells. Although many effectors of chromatin compaction have been reported, the nature and design of the essential trigger for global chromosome assembly remain unknown. Here we reveal the identity of the core mechanism responsible for chromosome morphogenesis in early mitosis. We show that the unique sensitivity of the chromosome condensation machinery for the kinase activity of Cdk1 acts as a major driving force for the compaction of chromatin at mitotic entry. This sensitivity is imparted by multisite phosphorylation of a conserved chromatinbinding sensor, the Smc4 protein. The multisite phosphorylation of this sensor integrates the activation state of Cdk1 with the dynamic binding of the condensation machinery to chromatin. Abrogation of this event leads to chromosome segregation defects and lethality, while moderate reduction reveals the existence of a novel chromatin transition state specific to mitosis, the intertwist configuration. Collectively, our results identify the mechanistic basis governing chromosome morphogenesis in early mitosis and how distinct chromatin compaction states can be established via specific thresholds of Cdk1 kinase activity.

SMC complexes differentially compact mitotic chromosomes according to genomic context

“…In situ hybridization of individual loci has suggested that a similar process could occur on chromosome arms 17,18,24 . Cohesin only accumulates on chromatin in S and early mitosis, while condensin is activated by CDK in S and M phase 15,19,35,36 . Therefore, both SMC complexes are relatively inactive in G1 and active in M, consistent with both these complexes promoting chromosome compaction in mitosis.…”

“…Smc4 phospoS4 antibody was a kind gift from Damien D’Amours 35 . Anti-HA antibody (12Ca5 mouse monoclonal IgG 2βK .…”

Mitotic p21Cip1/CDKN1A is regulated by cyclin-dependent kinase 1 phosphorylation

“…Interfering with the phosphorylation at S130 leads also to defective segregation and prolonged mitotic duration suggesting that a precisely controlled functional p21 ensures an errorless progression through mitosis by fine-tuning Cdk1 activity. Cdk1/cyclin B1 controls various key steps in early mitosis like bipolar spindle assembly [41] and chromosome condensation [42] and its timely inactivation is essential for the initiation of anaphase and contributing to its irreversibility [43]. Defects in those events result in failed chromosome segregation in anaphase.…”

Quantitative phosphoproteomics reveals new roles for the protein phosphatase PP6 in mitotic cells

“…The identification of PP6c–regulated phosphorylation sites on proteins linked to chromosome condensation is intriguing. Protein phosphorylation is an important regulatory mechanism in chromosome condensation, and the integration of specific kinase activities in both interphase and mitosis is important for the fidelity of chromosome condensation (69, 77–82). Furthermore, the role of protein phosphatases in chromosome condensation and decondensation is just emerging (83–85).…”