Mechanical Softening of Microtubules by TRIM46

Microtubules provide structural support and transport pathways in neurons. At the axon initial segment (AIS), they form tightly packed, parallel bundles that help control what enters the axon. Yet, these naturally stiff filaments must withstand mechanical stress while permitting cargo passage. Here, we show that TRIM46, a protein known to organize AIS microtubule bundles, makes microtubules more flexible. In cultured neurons, elevated TRIM46 expression induced strongly curved and ring-shaped microtubule structures. Experiments with purified proteins revealed that TRIM46 reduced the bending stiffness of both individual microtubules and bundles. Remarkably, even GMPCPP microtubules, which are stiffer than physiological GTP microtubules, bent markedly under fluid flow with TRIM46. Furthermore, when driven by kinesin motors, TRIM46-cross-linked bundles underwent continuous dynamic shape changes, forming ring-shaped microtubule assemblies that recapitulated the structures observed in neuronal axons. These findings reveal a mechanical function of TRIM46, suggesting that flexible bundling accommodates both axonal transport and mechanical stress.