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.