Fig. 7. Molecular model for early signaling and Pelle activation in the Toll pathway. A membrane-localized Toll/Tube/Pelle complex forms, perhaps transiently, before signaling (Toll TIR and inhibitory domains and Pelle regulatory and catalytic domains are indicated). Upon binding of the Toll ligand Spätzle, a high local Pelle concentration, induced by the resulting Toll dimerization (broken lines), is created, and this leads to Pelle transphosphorylation (arrows) and activation. Activated Pelle then phosphorylates the Toll Tir domain and Tube (curved arrows), Pelle (and Tube) are released from Toll, and Pelle phosphorylates downstream targets. See text for additional details.