Eukaryotic genomes are partitioned into chromosomes that form compact and spatially well-separated mechanical bodies during mitosis1,2,3. This enables chromosomes to move independently of each other for segregation of precisely one copy of the genome to each of the nascent daughter cells. Despite insights into the spatial organization of mitotic chromosomes4 and the discovery of proteins at the chromosome surface3,5,6, the molecular and biophysical bases of mitotic chromosome structural individuality have remained unclear. Here we report that the proliferation marker protein Ki-67 (encoded by the MKI67 gene), a component of the mitotic chromosome periphery, prevents chromosomes from collapsing into a single chromatin mass after nuclear envelope disassembly, thus enabling independent chromosome motility and efficient interactions with the mitotic spindle. The chromosome separation function of human Ki-67 is not confined within a specific protein domain, but correlates with size and net charge of truncation mutants that apparently lack secondary structure. This suggests that Ki-67 forms a steric and electrostatic charge barrier, similar to surface-active agents (surfactants) that disperse particles or phase-separated liquid droplets in solvents. Fluorescence correlation spectroscopy showed a high surface density of Ki-67 and dual-colour labelling of both protein termini revealed an extended molecular conformation, indicating brush-like arrangements that are characteristic of polymeric surfactants. Our study thus elucidates a biomechanical role of the mitotic chromosome periphery in mammalian cells and suggests that natural proteins can function as surfactants in intracellular compartmentalization.
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We thank the IMBA/IMP BioOptics microscopy facility and Molecular Biology Service, J. Meissner, and M. J. Hossain for technical support, H. Liu and S. Tietscher for generation of plasmids, C. Haering, M. Samwer, W. H. Gerlich, and O. Wueseke for comments on the manuscript, Life Science Editors for editing assistance, and U. Kutay for LAP2β-GFP/H2B–mRFP-expressing cells. D.W.G., A.A.H. and J.E. have received funding from the European Community’s Seventh Framework Programme FP7/2007-2013 under grant agreement no. 241548 (MitoSys), and A.Z.P., D.W.G. and J.E. under grant agreement no. 258068 (Systems Microscopy). D.G. has received funding from an ERC Starting Grant under agreement no. 281198 (DIVIMAGE), and from the Austrian Science Fund (FWF) project no. SFB F34-06 (Chromosome Dynamics). S.C. has received funding from a Human Frontier Science Program Long-Term Postdoctoral Fellowship and the European Community’s Seventh Framework Programme FP7/2007-2013 under grant agreement no. 330114 (IEF). T.M.R. was supported by Deutsche Forschungsgemeinschaft (DFG): SPP1384 ‘Mechanisms of Genome Haploidization’ MU 1423/3-2 and grant MU 1423/8-1.
Extended data figures
Extended data tables
Mitotic monoclonal fluorescent HeLa cell line expressing H2B-mCherry/CENPA-EGFP was imaged in presence of nocodazole 70 h after indicated siRNA transfection. Bar, 10 µm.
Monoclonal fluorescent HeLa cell line expressing H2B-mRFP/LAP2β-EGFP was imaged in the presence of nocodazole 48 h after indicated siRNA transfection. Bar, 10 µm.
Ki-67 knock-out HeLa cell line labeled with SiR-Hoechst was imaged in presence of nocodazole starting at prophase. Bar, 10 µm.
About this article
Scientific Reports (2018)