Classical swine fever virus recruits ALIX and ESCRT-III to facilitate viral budding

Classical swine fever virus (CSFV) incurs substantial economic losses in the global swine industry due to its persistent emergence and re-emergence across various countries. However, the precise mechanisms governing CSFV budding remain inadequately understood. Our study elucidates that the endosomal sorting complex required for transport (ESCRT)-associated protein ALIX, in conjunction with ESCRT-III, plays a pivotal role in orchestrating CSFV budding. Genomic sequence analysis identified a critical interaction between the YPXnL late domain on the E2 protein and ALIX. Through immunoprecipitation and structural domain deletion assays, we demonstrated that the ALIX Bro1 domain specifically recognized viral particles by binding to the YPXnL motif. Immunoelectron and transmission electron microscopy further confirmed that, upon infection, ALIX accumulated at the periphery of subcellular organelles, including COPII vesicles, endosomes, and the Golgi apparatus, thereby facilitating CSFV budding. Our findings also revealed that ESCRT-III subunits CHMP2B, CHMP4B, CHMP7, and VPS4A interacted with ALIX to expedite CSFV budding. Notably, Rab8 activated by Kif4A contributed to the release of CSFV particles by interacting with ALIX and directing ALIX-containing vesicles along microtubules toward the cytosol. Our study demonstrates that ALIX specifically recognizes E2 and orchestrates the recruitment of ESCRT-III and Rab8 to facilitate the vesicular budding of CSFV particles from the Golgi apparatus to the cytosol. Ultimately, virus-laden vesicles propelled by Kif4A are transported along microtubules to the plasma membrane for release. Our findings offer the first comprehensive elucidation of the CSFV budding process and contribute to the identification of antiviral targets, thereby advancing the development of antiviral therapeutics.IMPORTANCEThe endosomal sorting complex required for transport (ESCRT) machinery plays a pivotal role in the sorting of membrane proteins in eukaryotic cells and regulating various stages of infection for numerous viruses. Previous studies have underscored the indispensable role of ESCRT in the cellular entry and replication of classical swine fever virus (CSFV). However, the precise mechanisms by which ESCRT recognizes CSFV particles and initiates viral vesicle budding have remained elusive. This study reveals that the Bro1 domain of ALIX initiates viral budding proximal to the Golgi apparatus by specifically recognizing the YPXnL late domain on the CSFV E2 protein. Mechanistically, ALIX and ESCRT-III facilitate Rab8-regulated endosomal transport of CSFV particles from the Golgi apparatus to the plasma membrane. Subsequently, virions are propelled by the kinesin Kif4A along microtubules for egress into the extracellular space. In summary, these findings significantly advance our understanding of CSFV pathogenesis and offer valuable insights into the vesicular transport and budding mechanisms of CSFV particles.