Human cell division involves hundreds of proteins at its core. Knowing the 3D structure of these proteins is essential to understand how our genetic material is duplicated and transmitted through generations. The groups of Andrea Musacchio and Stefan Raunser at the Max Planck Institute for Molecular Physiology in Dortmund are now able to reveal the first detailed structure of a key protein complex for human cell division known as CCAN. Using cryo-electron microscopy, the researchers show important features of the complex’s 16 components and challenge previous assumptions about how the complex is able to recognize the centromere, a crucial region of chromosomes in cell division.
At the center of cell division
The centromere is a constriction of the chromosome, made up of DNA and proteins. More importantly, the centromere is the dock for the kinetochore, a machinery of about 100 proteins that results in the separation of two identical chromosomes during cell division and their delivery to daughter cells. Previous research has shown that the kinetochore docks to the centromere via the CCAN complex: CCAN interacts with centromere protein A, the centromere landmark protein. CCAN is also responsible for replenishing centromere protein A once cell division has taken place. Yet the details of the interaction between CCAN and centromere protein A remain elusive. “Understanding how CCAN recognizes and binds to the centromere could potentially lead us to build a centromere from scratch,” says Musacchio. The centromere is a major obstacle for synthetic biologists who aim to design artificial chromosomes to restore missing functions or introduce new ones into cells.
Unresolved issues at heart
Scientists identified the CCAN complex more than 15 years ago. “Yet building a pipeline to synthesize all the proteins in vitro was a major hurdle,” Musacchio says. After obtaining a first reconstruction of the human CCAN complex in vitro, Musacchio teamed up with Stefan Raunser, also at MPI Dortmund, who applied cryo-electron microscopy to the entire CCAN protein complex.
In the new publication, the MPI groups were able to determine the 3D structural details of the human CCAN complex, highlighting its unique characteristics and the implications for an interaction with centromere protein A. “Contrary to what was expected, this structure does not recognize centromere protein A directly into the standard configuration,” says Musacchio. Centromere protein A is most often packed with DNA and other proteins in the form of a nucleosome, the standard unit of genetic material. The authors now suggest that the Centromere protein A may be embedded in the centromere with a different configuration that may facilitate the crucial interaction with CCAN.They plan to identify the conditions that could lead to this new configuration and prove their hypothesis.