CASTOR2 restricts PRRSV internalization through AKT-mTOR signaling and interacting with viral glycoprotein GP5

Porcine reproductive and respiratory syndrome virus (PRRSV) causes substantial economic losses in the swine industry, yet the host determinants governing its entry and early infection remain incompletely defined. Here, we identified cytosolic arginine sensor for mTORC1 subunit 2 (CASTOR2) as a restriction factor against PRRSV. PRRSV infection markedly reduced CASTOR2 expression, whereas CASTOR2 overexpression suppressed viral replication and progeny production, and CASTOR2 depletion enhanced PRRSV infection. Mechanistically, CASTOR2 specifically blocks the internalization stage of viral endocytosis without affecting initial cell surface attachment. Intriguingly, PRRSV infection elicited the redistribution of CASTOR2 from its predominant cytoplasmic compartment to EEA1-positive early endosomes, providing a spatial basis for its activity during viral entry. CASTOR2 also interacted with the viral envelope glycoprotein GP5, suggesting a potential association with the viral entry machinery. Transcriptomic analysis further identified alterations in endocytosis- and PI3K-AKT-associated pathways following CASTOR2 expression. In parallel, PRRSV infection activated AKT/mTOR signaling, whereas CASTOR2 attenuated this activation; pharmacological activation of AKT or mTOR partially rescued PRRSV infection in CASTOR2-expressing cells. Together, these findings identify CASTOR2 as a host restriction factor that limits PRRSV infection by restricting viral internalization and modulating AKT/mTOR signaling, providing new insights into host-virus interactions during PRRSV entry.