Novel Arabidopsis microtubule-associated proteins track growing microtubule plus ends.
BACKGROUND
Microtubules (MTs) are polarized polymers with highly dynamic plus ends that stochastically switch between growth and shrinkage phases. In eukaryotic cells, a plethora of MT-associated proteins (MAPs) regulate the dynamics and higher-order organization of MTs to mediate distinct cellular functions. Plus-end tracking proteins (+TIPs) are a group of MAPs that specifically accumulate at the growing MT plus ends, where they modulate the behavior of the MT plus ends and mediate interactions with cellular targets. Although several functionally important + TIP proteins have been characterized in yeast and animals, little is known about this group of proteins in plants.


RESULTS
We report here that two homologous MAPs from Arabidopsis thaliana, Growing Plus-end Tracking 1 (GPT1) and GPT2 (henceforth GPT1/2), contain basic MT-binding regions at their central and C-terminal regions, and bind directly to MTs in vitro. Interestingly, GPT1/2 preferentially accumulated at the growing plus ends of cortical MTs in interphase Arabidopsis cells. When the GPT1/12-decorated growing plus ends switched to rapid depolymerization, GPT1/2 dissociated from the MT plus ends. Conversely, when the depolymerizing ends were rescued and started to polymerize again, GPT1/2 were immediately recruited to the growing MT tips. This tip tracking behavior of GPT proteins does not depend on the two established plant + TIPs, End-Binding protein 1 (EB1) and SPIRAL1 (SPR1).


CONCLUSIONS
The Arabidopsis MAPs GPT1 and GPT2 bind MTs directly through their basic regions. These MAPs track the plus ends of growing MTs independently of EB1 and SPR1 and represent a novel plant-specific + TIP family.