Both Membrane-Bound and Soluble Alkaline Phosphatases Bind Cry1Ca to Potentiate Its Toxicity in Spodoptera litura

Membrane-bound alkaline phosphatases (mALPs) are established receptors for several Bacillus thuringiensis (Bt) Cry toxins, yet the role of soluble ALPs (sALPs) in the insecticidal mechanism remains unclear. Here, we characterized two midgut-abundant ALPs in Spodoptera litura, SlALP8 and SlALP5, whose expression was down-regulated upon the exposure to Cry1Ca. SlALP8 is a glycosylphosphatidylinositol (GPI)-anchored protein, whereas SlALP5 is secreted. Co-immunoprecipitation assays confirmed that both proteins bind directly to Cry1Ca. RNA interference-mediated knockdown of either SlALP8 or SlALP5 expression increased the larval tolerance to Cry1Ca. Conversely, dietary supplementation with recombinant SlALP8 or SlALP5 protein significantly potentiated the toxicity of Cry1Ca. We demonstrate that membrane-bound SlALP8 acts as a receptor for Cry1Ca and soluble SlALP5 interacts with the toxin to potentiate its toxicity. These findings unveil a coordinated role of mALP and sALP in the Cry toxin efficacy, providing new insight into the mode of action of Cry toxins.