Stephen Harrison (Harvard Medical School, Boston, MA) and Peter Sorger (MIT, Cambridge, MA) are hoping to analyze the structure of the 60 or more yeast kinetochore proteins one subcomplex at a time. The current success with the 10-protein DASH complex, Harrison says, “came about on a dare to see if [first author JJ Miranda] could coexpress the whole thing in E. coli.” Happily, the bold experiment worked, the purified complex bound MTs, and the electron micrographs clearly showed rings of DASH complex encircling an MT.
The ring structure immediately suggests a mechanistic possibility. “A major way in which evolution has made entities processive is by making rings,” says Harrison. In this case, the outward splaying of MT protofilaments as the end of an MT falls apart should keep the ring on the intact section of the MT. This would effectively translocate the ring and thus the attached kinetochore towards the pole-attached end of the shrinking MT. Indeed, Stefan Westermann and Georjana Barnes (University of California, Berkeley, CA) used Miranda's construct to not only come up with a similar structure, but also to gain evidence for mobility of DASH rings along MTs.