SSNA1 mechanically reinforces the damaged microtubule lattice

Abstract

SSNA1 (Sjogrens Syndrome Nuclear Autoantigen 1) is a microtubule-associated protein involved in key cellular processes, including cell division, intraflagellar transport, and axonal branching. SSNA1 specifically localizes to sites of damage along the microtubule lattice, thus acting as a microtubule damage sensor. However, the effects of SSNA1 on microtubule mechanics or on the process of microtubule self-repair, which involves the incorporation of soluble tubulin dimers into lattice damage sites, are not known. Here, we use in vitro reconstitution with purified proteins and total internal reflection fluorescence (TIRF) microscopy to probe SSNA1s effects on microtubule mechanics and self-repair. We apply two distinct sources of force to investigate microtubule mechanics: kinesin-driven gliding assays and microfluidic flow. We find that SSNA1 binding increases microtubule rigidity and resistance to breakage under the physiological and controlled forces in our assays. Interestingly, SSNA1s localization to microtubule damage sites prevents the incorporation of new tubulin dimers and thus inhibits lattice self-repair. Conversely, we find that SSNA1 does not recognize damage sites that have been repaired by tubulin incorporation. Together, our findings demonstrate that SSNA1 reinforces the mechanical strength of microtubules without promoting self-repair, suggesting an alternative mechanism for restoring microtubule integrity in the absence of tubulin-mediated repair and providing new insights into SSNA1s mechanism of microtubule stabilization. Significance StatementMicrotubules are cytoskeletal polymers that experience mechanical stress during essential cellular processes such as cargo transport, cell division, and ciliary beating. To maintain their integrity, microtubules rely on both stabilizing proteins and repair mechanisms. Here, we show that microtubule-associated protein SSNA1 strengthens microtubules by increasing their rigidity and resistance to force-induced breakage, while simultaneously blocking tubulin-mediated lattice repair at sites of damage. By distinguishing between damaged and repaired microtubule lattices, SSNA1 enforces a stabilization strategy that favors mechanical reinforcement over self-repair. These findings reveal a new mode of microtubule regulation that decouples mechanical stability from lattice repair and provide insight into how cells preserve cytoskeletal integrity under force.