Mechanism of microtubule plus-end tracking by the plant-specific SPR1 protein and its development as a versatile plus-end marker

The dynamics and functions of microtubule plus-ends are governed by microtubule plus-end tracking proteins (+TIPs). Here, we report that the diminutive Arabidopsis thaliana SPIRAL1 (SPR1) protein, which regulates directional cell expansion, is an autonomous +TIP. Using in vitro reconstitution experiments and total internal reflection fluorescence microscopy, we demonstrate that the conserved N-terminal domain of SPR1 and its GGG motif are necessary for +TIP activity, whereas the conserved C-terminal domain and its PGGG motif are not. In addition, we show that the N-and C-terminal domains, either separated or in tandem, are sufficient for +TIP activity and do not significantly perturb microtubule plus-end dynamics compared to full-length SPR1. We also found that exogenously expressed SPR1-GFP and NC-GFP label microtubule plus-ends in animal cells. These data establish SPR1 as a new type of intrinsic +TIP and demonstrate its utility as a universal microtubule plus-end marker.