School of Physics Soft Condensed Matter & Biophysics Seminar: Prof. Jennifer Ross, University of Massachusetts Amherst

Proper organization of the microtubule cytoskeletal network is required to perform many necessary cellular functions including mitosis, cell development, and cell motility. Network organization is achieved through filament remodeling by microtubule-associated proteins (MAPs) that control microtubule dynamics. MAPs that stabilize are relatively well understood, while less is known about destabilizing MAPs, such as the family of microtubule-severing enzymes. Katanin, the first-discovered microtubule-severing enzyme, is a AAA+ enzyme that oligomerizes into hexamers and uses ATP hydrolysis to sever microtubules. Using quantitative fluorescence imaging on reconstituted microtubule severing assays in vitro we investigate how katanin can regulate microtubule dynamics. Interestingly, we find microtubule dynamics inhibits katanin severing activity - dynamic microtubules are not severed. This is surprising, because katanin appears to act on dynamic microtubules in the cell. Using systematic experiments, we find that free tubulin can compete for microtubule filaments for the katanin proteins.  Our results are even more curious because the cell is full of free tubulin dimers. Our work indicates that katanin activity is regulated by tubulin dimer concentration, which is a product of severing. Thus, a negative feedback loop works to shut down severing, to protect the microtubule cytoskeleton from total obliteration through severing.