Patronin promotes the formation of dynamic microtubule seeds in vivo

Microtubule seeds are short, uncapped microtubule fragments that can reinitiate growth. Whether they exist in cells in vivo has not been demonstrated. In Drosophila neurons, many microtubules are dynamic at both ends. When tracking the behavior of these microtubules in vivo, we found that microtubule seeds were frequently formed when microtubules depolymerized completely (end-to-end, E2E). The seeds most often repolymerized a new microtubule in the same orientation but could reverse polarity or be transported. The minus-end-tracking protein Patronin localized to these seeds and promoted seed formation. We hypothesized that Patronin might promote seed formation through its known role of enhancing minus-end growth. However, microtubule seed formation did not require minus-end growth and Patronin levels did not influence the probability that a minus end would be growing at the time of E2E depolymerization. Instead, Patronin was enriched on the microtubule coincident with the plus end meeting the minus end. We propose that Patronin promotes seed formation through a function separate from its role in minus-end growth, perhaps by recognizing a structure specific to a short microtubule fragment.