Kevin Roach
Research: Background: Accurate chromosome segregation is essential to all eukaryotes. This vital function is mediated by the kinetochore complex of proteins, which binds to the centromere and provides the attachment site for microtubules. It is the microtubules that pull apart the sister chromatids, ensuring proper segregation. Incorrectly attached or unattached microtubules trigger a cascade of signals that halt cell division. Despite stringent functional constraints on the process of chromosome segregation, the underlying DNA sequences that define centromeric regions are poorly conserved and evolve quite rapidly even between closely related species. Cenp-A and Cenp-C are the two kinetochore proteins that recognize and bind centromeric DNA, found in all eukaryotes. Disruption or loss of Cenp-C results in segregation defects. Cenp-C varies from 549 to 943 amino acids in length and is characterized by one conserved 24 amino acid domain near its carboxyl terminus. The centromere targeting and DNA binding domains of primate Cenp-C are known and mapped in the carboxyl two thirds of the protein. I have cloned and sequenced Cenp-C cDNA from primates representing 35 million years of divergence. Whereas one might expect an essential kinetochore protein to evolve under stringent evolutionary constraint, I instead found strong signatures of positive selection throughout the Cenp-C protein. Positive selection (when the rate of non-synonymous substitutions in the gene exceeds the rate of synonymous substitutions) is typically only seen in proteins involved in host-pathogen interactions. Cenp-C is involved in a critical cellular process that must take place perfectly at every cell division, so it is extremely surprising to see any signs of positive selection. My aim is to understand the causes of this positive selection and its consequences on the process of chromosome segregation. I am investigating two hypotheses to explain the rapid evolution of Cenp-C. Hypothesis 1: Cenp-C is rapidly evolving to respond to changes in centromere sequence. Female meiosis is an asymmetric cell division. Only one of the four products of female meiosis becomes the oocyte and is passed to the next generation whereas the other three are evolutionary dead-ends. Changes in centromere sequence or size can alter the frequency at which a particular chromosome is transmitted in female meiosis. This process would lead to rapid changes in centromere sequence as chromosomes compete for success. This competition, in turn, increases pressure on centromere binding proteins to adapt, alter binding, and restore meiotic parity, by suppressing the advantage of the ‘selfish’ centromere ( Henikoff, S. et al. Nature 2002 417:227 ). Hypothesis 2: Cenp-C is rapidly evolving to evade viral factors. Cenp-C’s critical role in the cell cycle and chromosome segregation has made it a target for viruses that aim to subvert the cell cycle. The Herpes Simplex Virus (HSV)-1 immediate-early protein ICP0 specifically recognizes and directs the proteasome-mediated destruction of human Cenp-C ( Everett, RD et al. EMBO J 1999 18:1526-38 ). Destruction of Cenp-C causes mitotic arrest, which may benefit the virus by conserving resources for virion production. This facilitates the efficient reactivation of the HSV-1 virus from latency. Stop ping the degradation of Cenp-C would be advantageous to the host because the cell would continue to divide properly and reduce the exclusive access of HSV-1 to resources. These two hypotheses are not mutually exclusive. I have found signatures of positive selection consistent with each hypothesis, in both the ICP0 targeted domains as well as the centromere targeting domains of Cenp-C. Summary: The finding that a protein critical for proper chromosome segregation is changing rapidly is highly unexpected, but valuable to our understanding of this fundamental process. The complex interactions between Cenp-C, centromeric DNA and viral antagonists like ICP0 will require a combination of evolutionary and cellular biology to untangle. The dividends of this interdisciplinary approach will be an enhanced understanding one of cell biology’s most interesting paradoxes (rapid evolution of centromeric proteins despite conserved function).
|
Joined Program: 2006