Patronin Regulates the Microtubule Network by Protecting Microtubule Minus Ends
Microtubules are the principle scaffold of the mitotic spindle, serve as tracks for intracellular transport of proteins and mRNAs, and also participate in signaling functions. Microtubules are polymers of alpha/beta tubulin heterodimers, which polymerizes in a head-to-tail to form polar filaments. In vitro both the plus and minus end of the microtubule undergo dynamic instability, whereby the microtubule undergoes prolonged periods of growth and shrinkage, with infrequent transitions between the two. In vivo, the plus end of the microtubule is very dynamic, and many proteins have been identified that bind at microtubule plus ends and regulate dynamicity. However, in contrast to the wealth of information on the microtubule plus end, the regulation of the microtubule minus end in vivo is poorly understood. Minus ends are mostly static within the cell, suggesting that microtubule minus ends might be capped by some unknown protein(s) that suppresses subunit dynamics.
In a whole-genome RNAi screen for spindle morphology defects in Drosophila S2 cells, I identified a previously uncharacterized protein whose depletion caused short spindles in mitosis and microtubule fragments in interphase. This protein, which we have named Patronin for the Latin `patronus' (protector), protects microtubule minus ends in vivo from depolymerization by Kinesin-13. In the absence of Patronin, microtubules release from their nucleating sites and treadmill through the cytoplasm, a result of unhindered minus end depolymerization. Purified Patronin selectively binds to and protects minus ends from Kinesin-13-induced depolymerization in vitro, demonstrating that Patronin alone is sufficient to confer minus end stability. Thus, Patronin is the first protein shown to cap and stabilize the microtubule minus end in vivo. Furthermore, we demonstrate that microtubule minus end dynamics are regulated by competing actions of destabilizing and stabilizing proteins, as has been shown previously for the plus end. We also identify Patronin interaction partners and provide evidence suggesting that Patronin may be involved in scaffolding centrosomal proteins at microtubule nucleation sites throughout the cytoplasm.