UBA1 knockdown dysregulates UBA1-sensitive proteins and impairs muscle function in models of spinal muscular atrophy X-linked 2.

UBA1 is the primary ubiquitin-activating enzyme that initiates ubiquitination, which regulates protein function and turnover. Although UBA1 loss is cell lethal, silent mutations that reduce UBA1 mRNA levels cause spinal muscular atrophy X-linked 2 (SMAX2), a disorder marked by skeletal muscle weakness and wasting. However, it remains unexplored how UBA1 impacts the muscle proteome, and whether muscle weakness can arise from reducing UBA1 function solely in skeletal muscle. Here, we examined Drosophila and mice with muscle-targeted UBA1 knockdown and found that this intervention reduced protein ubiquitination, muscle function and lifespan. Integrated transcriptomic and proteomic analyses indicated that a limited set of proteins is modulated post-transcriptionally by Uba1 RNA interference (Uba1RNAi), suggesting that these UBA1-sensitive proteins rely on optimal UBA1 levels for degradation (Uba1RNAi-upregulated proteins) and stability (Uba1RNAi-downregulated proteins). Therefore, despite the general function of UBA1 in ubiquitination, UBA1 knockdown alters the levels of relatively few critical proteins, which may contribute to muscle weakness and SMAX2 pathogenesis. Moreover, although SMAX2-linked UBA1 mutations occur ubiquitously, experimental reduction of UBA1 function solely in skeletal muscle recapitulates key disease aspects, highlighting a possible muscle-centric origin of SMAX2.