Stimulating microtubule growth is not the essential function of the microtubule polymerase Stu2

Abstract

ABSTRACTTOG family proteins, including the budding yeast Stu2, are essential for the formation of a functional mitotic spindle. Across all eukaryotes, the described functions of this family depend on two microtubule binding elements: TOG domain arrays, and a basic linker domain important for binding the microtubule lattice. Consistently, we find here that Stu2’s basic linker is required for its ability to regulate microtubules in vitro, including stimulating microtubule growth, shrinkage, and catastrophe. We furthermore define a region contained within Stu2’s basic linker domain as its nuclear localization sequence, and identify phospho-regulation that promotes mitosis-specific nuclear import. Surprisingly, directing nuclear localization is the only function contained within Stu2’s basic linker that is required for cell viability, indicating that microtubule lattice binding is not required for Stu2’s essential function. Considering that lattice binding is required to stimulate microtubule polymerization and depolymerization in vitro, these established activities are unlikely to be the essential functions carried out by Stu2 in the cell’s nucleus.SUMMARYStu2 is a TOG family protein that performs numerous microtubule regulatory functions in the cell. Here we show that Stu2’s nuclear localization is essential for cell viability. Surprisingly, its required nuclear function is distinct from its canonical activities regulating microtubules.