A divergent TOG domain and oligomerization contribute to SPIRAL2 function in stabilizing microtubule minus ends.

The acentrosomal cortical microtubules of higher plants dynamically assemble into specific array patterns that determine the axis of cell expansion. Recently, the Arabidopsis (Arabidopsis thaliana) SPIRAL2 (SPR2) protein was shown to regulate cortical microtubule length and light-induced array reorientation by stabilizing microtubule minus ends. SPR2 autonomously localizes to both the microtubule lattice and microtubule minus ends, where it decreases the minus end depolymerization rate. However, the structural determinants that contribute to the ability of SPR2 to target and stabilize microtubule minus ends remain unknown. Here, we present the crystal structure of the SPR2 N-terminal domain, which reveals a unique tumor overexpressed gene (TOG) domain architecture with seven HEAT repeats. We demonstrate that a coiled-coil domain mediates multimerization of SPR2 that provides avidity for microtubule binding and is essential to bind soluble tubulin. In addition, we found that a SPR2 construct spanning the TOG domain, basic region, and coiled-coil domain targets and stabilizes microtubule minus ends similar to full-length SPR2 in plants. These results reveal how a TOG domain, which is typically found in microtubule plus-end regulators, has been appropriated in plants to regulate microtubule minus ends.