A DltE-DltD-DltX interaction network regulates lipoteichoic acid D-alanylation and Lactiplantibacillus plantarum-mediated growth promotion in Drosophila.

D-alanylation of teichoic acids is a conserved modification of Gram-positive bacterial cell envelopes that modulates resistance to environmental stresses and host interactions. While the cytosolic steps of this pathway are well characterized, the extracellular reactions responsible for transferring D-alanine onto teichoic acids remain poorly understood. Here we investigate the organization of the Dlt machinery in the symbiotic bacterium Lactiplantibacillus plantarum. We determined the 2.3 Å crystal structure of the extracellular catalytic domain of DltD, which adopts an SGNH-hydrolase fold with a conserved Ser-His-Asp catalytic triad. Docking analyses with lipoteichoic acids (LTA) fragments suggest that the glycerol-phosphate backbone of LTA is accommodated along a surface groove leading to the catalytic serine, with conserved residues contributing to substrate positioning. Biochemical measurements further reveal direct interactions between DltD, the acyl-carrier protein DltX, and the LTA esterase DltE. The conserved C-terminal motif of DltX binds DltD and is required for efficient D-alanylation and for L. plantarum-mediated promotion of Drosophila juvenile growth. Together, our findings support a revised model in which DltD, DltE, and DltX form a coordinated extracellular interaction network that dynamically regulates LTA D-alanylation. This work provides new mechanistic insights into the organization of the Dlt machinery and reveals how species-specific adaptations of this conserved pathway contribute to bacterial interactions with the host.