Microtubule plus-end dynamics are tightly coupled to the turnover of the MOR1 polymerase

MOR1/XMAP215/Dis1/Stu2 family proteins function as polymerases that promote microtubule assembly through tubulin-interacting N-terminal tumor-overexpressed gene (TOG) domains. It remains unclear how the polymerase activity of these proteins is coordinated with their movement along the growing microtubule lattice. Previously, it was found that a L174F mutation in the N-terminal TOG domain of the 217 kDa Arabidopsis thaliana MICROTUBULE ORGANIZATION 1 (MOR1) protein impaired microtubule growth and shrinkage rates in a temperature-dependent manner. In this study, we used a homologous recombination strategy to generate stable transgenic lines expressing full-length MOR1 and mutant mor1-1 ^L174F proteins tagged with the yellow fluorescent protein YPet. When expressed in mor1 mutant backgrounds, MOR1-3xYPet showed strong association with both the growing and shrinking microtubule plus ends, consistent with its involvement in both polymerization and depolymerization. By contrast, when expressed in a wild-type background, MOR1-3xYPet was redistributed along the microtubule lattice, microtubule growth rates were reduced, and plant growth was stunted. By quantifying fluorescence recovery after photobleaching, we confirmed that MOR1 has greater affinity for the plus end than the lattice. Exploiting the FRAP technique further, we identified a positive and linear correlation between microtubule polymerization rates and MOR1 turnover at the microtubule plus end. When microtubule growth rates were reduced in the mor1-1-YPet line by shifting the temperature from 21 to 30°C and for MOR1-3xYPet under taxol treatments, MOR1’s turnover was delayed. In contrast, microtubule growth rates and MOR1 turnover were hastened at 30°C or after CRISPR/Cas9-mediated knock out of the CLASP protein. These findings support a new model whereby MOR1 has finite processivity on the microtubule, catalyzing a limited fixed number of tubulin additions before dissociation.