Characterization of Spz5 as a novel ligand for Drosophila Toll-1 receptor

The Drosophila Toll-1 receptor is involved in embryonic development, innate immunity, and tissue homeostasis. Currently, as a ligand for the Toll-1 receptor, only Spätzle (Spz) has been identified and characterized. We previously reported that Drosophila larva-derived tissue extract contains ligand activity for the Toll-1 receptor, which differs from Spz based on the observation that larval extract prepared from spz mutants possessed full ligand activity. Here, we demonstrate that Spz5, a member of the Spz family of proteins, functions as a ligand for the Toll-1 receptor. Processing of Spz5 by Furin protease, which is known to be important for ligand activity of Spz5 to Toll-6, is not required for its function to the Toll-1 receptor. By generating a spz5 null mutant, we further showed that the Toll-1 ligand activity of larva-derived extract is mainly derived from Spz5. Finally, we found a genetic interaction between spz and spz5 in terms of developmental processes. This study identified a novel ligand for the Drosophila Toll-1 receptor, providing evidence that Toll-1 is a multi-ligand receptor, similar to the mammalian Toll-like receptor.