Septin 2/6/7 complexes tune microtubule plus end growth and EB1 binding in a concentration- and filament-dependent manner

Septins are filamentous GTP-binding proteins, which affect microtubule (MT) dependent functions including membrane trafficking and cell division, but their precise role in MT dynamics is poorly understood. Here, in vitro reconstitution of MT dynamics with SEPT2/6/7, the minimal subunits of septin heteromers, shows that SEPT2/6/7 has a biphasic concentration-dependent effect on MT growth. Lower concentrations of SEPT2/6/7 enhance MT plus end growth and elongation, while higher and intermediate concentrations inhibit and pause plus end growth, respectively. We show that SEPT2/6/7 has a 1.5-fold preference for GTP-over GDP-bound MT lattice, and competes with EB1 for binding to GTPγS-stabilized MTs, which mimic the EB1-preferred GDP-Pi state of polymerized tubulin. Strikingly, SEPT2/6/7 triggers EB1 dissociation from plus end tips in cis by binding to the MT lattice and in trans when MT plus ends collide with SEPT2/6/7 filaments. At these intersections, SEPT2/6/7 filaments were more potent barriers than actin filaments in pausing MT growth and dissociating EB1 in vitro and in live cells. These data demonstrate that SEPT2/6/7 complexes and filaments can directly impact MT plus end growth and the tracking of plus end-binding proteins, and thereby may facilitate the capture of MT plus ends at intracellular sites of septin enrichment. Highlight Summary for eTOC Knowledge of septin roles in MT dynamics is poor and confounded by knockdown studies. Here, in vitro reconstitution assays show concentration-dependent effects of SEPT2/6/7 on MT plus end growth, pausing and EB1 tracking. We found that SEPT2/6/7 filaments are potent than actin in pausing MT growth and dissociating EB1 from intersecting plus ends.