Cell division is the basis of life and requires that each daughter cell receive the proper complement of chromosomes. In most organisms, this process is mediated at the familiar constricted intersection of X-shaped chromosomes. This area, called the centromere, is where special proteins gather and attach to pull daughter cells apart during cell division. The structure and biology of the centromere is of considerable scientific interest because problems with it can lead to abnormalities in the chromosomes of daughter cells, which are the basis of such disorders as Down syndrome.

A new study by researchers at the Perelman School of Medicine at the University of Pennsylvania published in Science this week describes how the centromere is stabilized during replication. DNA in the nucleus is packaged into protein/DNA complexes called nucleosomes. As it turns out, the centromere is distinguished not only by its DNA sequence but also by a special type of nucleosome, which includes a protein called CENP-A. 

Senior author Ben E. Black PhD, associate professor of Biochemistry and Biophysics, and his Penn team described the structure of CENP-A almost five years ago. The question the investigators asked now was, how does the cell ensure that CENP-A-containing nucleosomes remain at, and thus continue to mark, the centromere during the massive changes a cell undergoes when it divides? 

Simply put, it involves an accessory protein called CENP-C. “Overall, my lab is interested in better understanding the molecular basis of inheritance and the role of the centromere, as a ‘control locus,’ for maintaining heredity,” says Black. 

His team applied a battery of biophysical techniques to study the structure and stability of CENP-A-containing nucleosomes in a test tube. Their data indicate that CENP-A-bearing nucleosomes have an unexpectedly flexible structure, adopting a relaxed conformation in the absence of CENP-C, and a more compact shape in its presence. This CENP-C-induced shape shift correlates with changes in how DNA wraps around the centromere’s nucleosomes, making the structure similar to that found in living cells.

Their findings also address the question of the stability of CENP-A molecules at centromeres. Under normal conditions CENP-A binds centromeres and effectively never lets go. Indeed, when the authors tracked where proteins “reside” in live cells, they found that, unlike traditional nucleosomes that package the DNA throughout the rest of the chromosome, CENP-A-containing nucleosomes apparently never dissociate after newly generated CENP-A protein is first delivered to the centromere during a short time window following cell division. “The CENP-A is basically cemented at the centromere of origin,” Black explains. But in cells lacking CENP-C, CENP-A dissociates readily, suggesting that CENP-C binding to CENP-A is what imparts that stability.

Investigators have known for the past 20 years that part of chromosome inheritance is controlled by epigenetics, implicating the protein spools around which DNA is wound as the driving force, rather than what is encoded in the DNA sequence itself. Those spools are built of histone proteins, and chemical changes to these spool proteins can either loosen or tighten their interaction with DNA. This, in turn, alters a gene’s expression up or down. In the case of the centromere, it marks the site where spindle fibers attach independently of the underlying DNA sequence.

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