Tubulin regulates stability and localization of STMN2 by binding preferentially to its soluble form

The small, tubulin-binding protein STMN2 is highly expressed in neurons and is implicated in amyotrophic lateral sclerosis. STMN2 degrades rapidly and accumulates at axotomy sites, suggesting fast turnover is crucial for its neuroprotective function. We show that STMN2 was primarily degraded by the ubiquitin-proteasome system. Its membrane-targeting N-terminal domain promoted fast turnover, whereas its tubulin-binding domain promoted stabilization. Proximity labeling and imaging showed that tubulin binding reduced STMN2 targeting to trans-Golgi network membranes. Pull-down assays showed that tubulin binds preferentially to soluble over membrane-bound STMN2. Our observations suggest that STMN2 interconverts between a soluble, tubulin-bound form and a membrane-bound, tubulin-free form, and is rapidly degraded when released from both membranes and tubulin. We propose that tubulin binding sequesters and stabilizes STMN2, while its neuroprotective function involves an unknown membrane activity.