The host protein SSR4 mediates PRRSV-induced endoplasmic reticulum stress via interaction with Nsp2.

Porcine reproductive and respiratory syndrome (PRRS) is a major economic burden to the global swine industry. Here, we identify the endoplasmic reticulum (ER) translocon component SSR4 as a critical host factor co-opted by PRRSV. We demonstrate that the viral non-structural protein Nsp2 physically interacts with SSR4 via its PLP2 and hypervariable domains and selectively upregulates its expression during infection by prolonging its protein half-life. Functional studies revealed that SSR4 is a proviral factor essential for efficient PRRSV replication. Mechanistically, SSR4 is required for the full activation of the PRRSV-induced ER stress response, specifically modulating the PERK-eIF2α and IRE1α-XBP1 axes of the unfolded protein response. Notably, Nsp2 itself acts as a key inducer of ER stress and mediates the upregulation of SSR4, suggesting a potential feed-forward loop that sustains a virus-favorable ER environment. This relationship is finely balanced, as pharmacological disruption of ER homeostasis using either the inducer tunicamycin (TU) or the chemical chaperone 4-phenylbutyric acid (4-PBA) potently inhibited viral replication. Importantly, TU and another inducer, dithiothreitol, exhibited potent, broad-spectrum antiviral activity against multiple PRRSV genotypes in both cell lines and primary porcine alveolar macrophages. Our study delineates a novel pathogenesis model where PRRSV Nsp2 hijacks SSR4 to engineer a proviral ER stress niche. The Nsp2-SSR4-ER stress axis represents a promising target for the development of broad-spectrum antiviral strategies against PRRS.IMPORTANCEThis study provides significant insights into porcine reproductive and respiratory syndrome virus (PRRSV) pathogenesis by identifying a novel and specific virus-host interface. We demonstrate that PRRSV, through its Nsp2 protein, hijacks a specific component of the host endoplasmic reticulum (ER) translocon SSR4 to orchestrate a tailored ER stress response conducive to viral replication. This mechanism is distinct from a general disruption of the TRAP complex, highlighting a precise viral strategy. Furthermore, the finding that pharmacological agents, which dysregulate this hijacked pathway-particularly ER stress inducers-act as potent, broad-spectrum antivirals challenges the conventional view of ER stress as a uniformly host-protective response. Our work not only uncovers a key molecular determinant of PRRSV replication but also validates the Nsp2-SSR4-ER stress axis as a promising and novel target for the development of much-needed, broad-spectrum therapeutic interventions against this economically devastating swine pathogen.