Researchers from the University of Osaka show that the CCAN protein complex forms a spherical shell-like structure in the cell nucleus, with the protein CENP-C indispensable for building it and for exact chromosome segregation

Osaka, Japan – Whenever a cell divides, it has to split its chromosomes perfectly between two daughter cells, and even a single mistake can be disastrous. Now, researchers from Japan have found that one of the key structures that makes this possible takes a surprising shape: a hollow sphere.

In a study to be published in Science Advances, researchers from the University of Osaka and their collaborators have revealed how a protein complex called the constitutive centromere-associated network (CCAN) assembles inside the cell, forming a spherical shell-like structure within the nucleus.

CCAN sits at the centromere, the pinched ‘waist’ of each chromosome, where it forms the basis of a larger structure called the kinetochore. The kinetochore acts like a tow hitch - it links each chromosome to the cellular ropes that pull chromosomes apart during cell division. Although previous studies have revealed a single CCAN unit’s structure in fine detail, each kinetochore contains dozens of these units, and consequently how they are arranged together inside cells has remained a mystery.

“If you know the shape of a single brick, that doesn’t tell you what the building looks like,” say three lead authors of the study Yuji Sakai, Mariko Ariyoshi, and Yasuhiro Hirano. “We wanted to know how multiple CCAN units combine to build a working kinetochore inside the cell.”

To achieve this, the team first used supercomputer simulations to model how multiple CCAN units behave when linked by a protein called CENP-C. The simulation results suggest that CENP-C clusters at the center while binding to DNA, which arranges other CCAN proteins into an orderly shell around it. The researchers then turned to expansion microscopy - a technique that physically swells cell samples so that tiny structures can be more clearly seen – as well as other high-resolution imaging methods. In keeping with the simulation predictions, a CCAN shell-like structure appeared in chicken cells, and it was most pronounced in the period leading up to cell division.

“When CENP-C is removed or prevented from binding to DNA, the shell is unable to form properly,” says senior author Tatsuo Fukagawa. “These cells also show defective chromosome segregation - this shows us that CCAN’s ordered shell-like structure is essential for faithful segregation of chromosomes.”

In addition to revealing a new feature of kinetochore architecture, the study shows the power of combining computer simulations with high-resolution imaging to probe structures that fall between the scale of individual molecules and whole cells.

Errors in chromosome segregation can leave cells with the wrong number of chromosomes, a common feature of many cancers. Therefore, understanding how the kinetochore is constructed could ultimately provide new insight into how such errors arise.
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The article, “CCAN forms the spherical shell-like structure in interphase kinetochores,” is to be published in Science Advances at DOI: https://doi.org/10.1126/sciadv.aef4365