A key component of a healthy cell is a complete and functioning genome. Thus, when one cell divides into two, it is careful to distribute its duplicated chromosomes evenly between its daughters. Failure in this process, known as chromosome segregation, can result in aneuploidy, a defect in chromosome number that commonly underlies miscarriage, congenital disorders such as Down Syndrome, and many cancers. During chromosome segregation, attached pairs of duplicated chromosomes, called chromatids, migrate to the center of the cell. Microtubules emanating from opposite sides of the cell then reach out and attach to these chromatids at specialized sites called kinetochores. Finally, motor proteins pull to reel in the microtubules, with the goal of pulling apart the chromatids and dragging one to each side of the cell before it splits in two. It’s a veritable game of molecular tug-of-war; microtubules compose the rope, motor proteins the players, and the referees – well, that’s where things get interesting. Before the chromatids are pulled apart, it’s important that kinetochores make so-called bioriented attachments, in which the two chromatids are attached to microtubules from opposite sides. The Aurora B protein is classically considered to ensure proper attachment. In the absence of a bioriented attachment “the lack of tension is believed to signal Aurora B kinase to phosphorylate kinetochore proteins, which weakens their grip on the microtubule, causing detachment and giving the cell another chance to make a proper attachment,” the authors write. But, as it turns out, Aurora B isn’t the only protein with this function. In a new paper in the Journal of Cell Biology, Drs. Lori Koch, Christian Nelson, and Sue Biggins of Fred Hutch’s Basic Sciences Division, in collaboration with Drs. Krishna Sarangapani and Chip Asbury of the University of Washington Department of Physiology and Biophysics, identify a new protein that referees this high-stakes game in which the only true win is a tie.
“The field has been focused on the role of the Aurora B kinase in phosphorylating kinetochores to help cells fix defective kinetochore-microtubule attachments,” explains Dr. Biggins. In their current work, however, her group turned their attention to another protein: Mps1. “The Mps1 kinase is another conserved essential kinase implicated in kinetochore biorientation and error correction,” they write. But what exactly Mps1 does at the kinetochore was not clear – does it regulate Aurora B? Does it act on its own? Existing evidence was conflicted on these questions, leading the authors to take a closer look and try to unravel the mystery of Mps1. To examine this protein’s function, they use a reconstitution system in which they could isolate kinetochores and study their properties in a simplified context in vitro. This approach is powerful in that it allows the authors to precisely measure the strength of microtubule-kinetochore attachments. Also, conveniently, their purified kinetochores contained Mps1 but not Aurora B, allowing them to ask what Mps1 can do all on its own. They found that activating Mps1 kinase activity significantly weakened microtubule-kinetochore attachments, revealing the protein’s role as a direct regulator of this process and suggesting that Mps1 and Aurora B act in parallel to promote bioriented attachment.
But this was not the end of the similarities between Mps1 and Aurora B. While Aurora B is known to act by phosphorylating the kinetochore protein Ndc80, Mps1 had been reported to phosphorylate another protein – Spc105. The authors tested which of these proteins was needed for Mps1-dependent microtubule-kinetochore attachment weakening in their reconstituted system, and found that in fact Ndc80, and not Spc105, was the Mps1 target. Looking more closely, they mapped Mps1 targeting to phosphorylation sites in the tail domain of Ndc80 – the very same domain targeted by Aurora B. Thus, Mps1 and Aurora B appear to be playing extremely similar roles in regulating microtubule-kinetochore attachment.
Finally, the group asked whether the results they observed in their in vitro system held up in vivo. Using an antibody they generated that recognizes phosphorylated Ndc80, they found that Mps1 does indeed phosphorylate this protein in cells whose kinetochores lack tension. They further reported that cells containing a mutant form of Ndc80 that could not be phosphorylated by Mps1 had defects in error correction, confirming their findings in vivo.
Dr. Biggins notes that the findings of this work are somewhat perplexing. “It is unclear why cells would use two different kinases to regulate the same domain of the same protein,” she said. But, she explained, the cell likely has its reasons: “[this could be] due to spatial or temporal differences depending on the type of kinetochore-microtubule attachment state or a redundancy to give extra protection”. Now, her aim is to better understand why such redundancy exists. “We will continue to explore the activity of the two kinases during the cell cycle to determine when they act as well as how tension on kinetochore-microtubule attachments regulates them.”
This work was supported by the National Science Foundation, the National Institutes of Health, the Howard Hughes Medical Institute, the David and Lucile Packard Foundation, and the Genomics and Scientific Imaging and the proteomics and metabolomics Shared Resources of the Fred Hutch/UW Cancer Consortium.
Fred Hutch/UW Cancer Consortium members Sue Biggins and Chip Asbury contributed to this work
Sarangapani KK, Koch LB, Nelson CR, Asbury CL, Biggins S. Kinetochore-bound Mps1 regulates kinetochore-microtubule attachments via Ndc80 phosphorylation. J Cell Biol. 2021 Dec 6;220(12):e202106130.