Spodoptera exigua Caspase-4 Enhances the Virulence of Baculovirus by Promoting Host Cell Apoptosis

Caspases play critical roles in virus-induced apoptosis. In lepidopteran insects, caspase-4 is a unique effector caspase, but its regulatory cascade and function during baculovirus infection remain unclear. Here, we identified and characterized caspase-4 from Spodoptera exigua (SeCaspase-4). Sequence analysis revealed high similarity to lepidopteran caspase-4 and Drosophila Damm. Enzyme activity and subcellular localization assays confirmed that SeCaspase-4 is an effector caspase that does not translocate to the nucleus and is functionally distinct from SeCaspase-1. Quantitative PCR revealed a sequential activation cascade: SeCaspase-5 activates SeCaspase-4, which in turn activates SeCaspase-1. Mutational analysis demonstrated that the large and small subunits and Cys287 are essential for proapoptotic activity of SeCaspase-4. Recombinant AcMNPV carrying SeCaspase-4 increased viral virulence against S. exigua larvae, presumably by promoting apoptosis in insect cells, despite reducing viral production. These findings clarify the lepidopteran apoptotic pathway and provide insights for engineering baculoviruses with improved insecticidal activity and agricultural application potential.