Abl2 mediates microtubule nucleation and repair via tubulin co-condensation

Abstract

SummaryAbl family kinases are evolutionarily conserved regulators of cell migration and morphogenesis. Genetic experiments in Drosophila suggest that Abl family kinases interact functionally with microtubules to regulate axon guidance and neuronal morphogenesis. Vertebrate Abl2 binds to microtubules and promotes their plus-end elongation both in vitro and in cells, but the molecular mechanisms by which Abl2 regulates microtubule (MT) dynamics were unclear. We report here that Abl2 regulates MT assembly via condensation and direct interactions with both the MT lattice and tubulin dimers. We find that Abl2 promotes MT nucleation, which is further facilitated by the ability of the Abl2 C-terminal half to undergo phase separation and form co-condensates with tubulin. A naturally occurring tubulin binding-deficient Abl2 splice isoform fails to promote nucleation. Abl2 binds to regions adjacent to MT damage and facilitates their repair via fresh tubulin recruitment and increases MT rescue frequency and lifetime. MT recovery after nocodazole treatment is greatly slowed in Abl2 knockout COS-7 cells compared to wild type cells. We propose a model in which Abl2 locally concentrates tubulin and recruits it to MT tips or to defects in the MT lattice to promote MT repair, rescue, and nucleation.

Spatially controlled microtubule nucleation and organization from crosslinker MAP65 condensates

“…Recently, it has been shown that condensed, liquid droplets of microtubule-associated proteins (MAPs), such as tau, TPX2, CAMPSAP2, and BuGZ, Abl2, and +Tip proteins, can form condensates via LLPS and nucleate microtubules when free tubulin is added to these condensates (21)(22)(23)(24)(25)(26)(27)(28). Tau is a good candidate for liquid-liquid phase separation (LLPS), since it is an intrinsically disordered protein (IDP) known to selfassociate (29,30), although it requires the addition of extra crowders to condense (22,23).…”