Ubiquitin-like protein UBL5 and spliceosome associated factor SART1 jointly maintain the virulence in Toxoplasma gondii.

The ubiquitin-like protein UBL5 plays conserved roles in pre-mRNA splicing across eukaryotes, yet its biological functions in the apicomplexan parasite Toxoplasma gondii remain unknown. Here, Our findings characterize TgUBL5 as an important regulator contributing to parasite proliferation, invasion, and acute virulence. Conditional depletion of TgUBL5 severely impairs tachyzoite growth, gliding motility, and host cell invasion, while abolishing pathogenicity during acute infection in BALB/c mice under IAA treatment. Co-immunoprecipitation assays reveal a direct interaction between TgUBL5 and the U5 snRNP-associated splicing factor TgSART1, whose degradation similarly disrupts parasite viability and virulence. RNA-seq profiling demonstrates that TgSART1 ablation triggers genome-wide splicing defects, predominantly intron retention, concomitant with transcriptional downregulation of key virulence effectors (ROP, MIC, RON, and SRS families). This SART1-dependent splicing defect phenocopies the invasion defect and virulence attenuation seen upon TgUBL5 depletion, suggesting that while TgSART1 directly regulates essential splicing, the virulence phenotype involves both factors, with TgUBL5 potentially stabilizing TgSART1. Our study uncovers a link between splicing regulation involving ubiquitin-like proteins and parasite infectivity.