SSNA1 (Sjögren’s syndrome nuclear autoantigen 1) is a microtubule-associated protein involved in key cellular processes, including cell division, intraflagellar transport, and axonal branching. Our previous work demonstrated that SSNA1 specifically localizes to sites of damage along the microtubule lattice, establishing it as a bona fide 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 SSNA1’s effects on microtubule mechanics and self-repair. We apply two distinct sources of force to investigate microtubule mechanics: microfluidic flow and kinesin-driven gliding assays. Our results show that SSNA1 binding increases microtubule rigidity and resistance to breakage under the physiological and controlled forces in our assays. Interestingly, we find that SSNA1’s 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, providing new insights into SSNA1’s mechanism of microtubule stabilization.
Richardson et al. (Sun,) studied this question.