Tao-1 is a negative regulator of microtubule plus-end growth.

Microtubule dynamics are dominated by events at microtubule plus ends as they switch between discrete phases of growth and shrinkage. Through their ability to generate force and direct polar cell transport, microtubules help to organise global cell shape and polarity. Conversely, because plus-end binding proteins render the dynamic instability of individual microtubules sensitive to the local intracellular environment, cyto-architecture also affects the overall distribution of microtubules. Despite the importance of plus-end regulation for understanding microtubule cytoskeletal organisation and dynamics, little is known about the signalling mechanisms that trigger changes in their behaviour in space and time. Here, we identify a microtubule-associated kinase, Drosophila Tao-1, as an important regulator of microtubule stability, plus-end dynamics and cell shape. Active Tao-1 kinase leads to the destabilisation of microtubules. Conversely, when Tao-1 function is compromised, rates of cortical-induced microtubule catastrophe are reduced and microtubules contacting the actin cortex continue to elongate, leading to the formation of long microtubule-based protrusions. These data reveal a role for Tao-1 in controlling the dynamic interplay between microtubule plus ends and the actin cortex in the regulation of cell form.