Identification and Structural Analysis of Apolipoprotein D with High Insecticide-Binding Affinity in <i>Spodoptera exigua</i>

The insecticide transport mechanisms in insects remain unclear. In <i>Spodoptera exigua</i>, we identified four secretory ApoD proteins. Structural modeling indicated that SeApoD1, SeApoD3, and SeApoD4 contain a conserved ligand-binding pocket accommodating lambda-cyhalothrin and chlorpyrifos with high affinity, while SeApoD2 exhibited a distorted cavity and negligible binding. The results of MicroScale Thermophoresis (MST) indicated that SeApoD3/4 had a high affinity for the two insecticides. Upregulation of three SeApoDs in a resistant strain implies a possible role in resistance, such as xenobiotic sequestration or tissue delivery. Our findings support a "molecular ferry" model for insecticide transport, broadening the functional scope of insect apolipoproteins and offering new insights into toxicodynamics and resistance evolution.