The proper alignment or polarity of the fly along its dorsal-ventral axis is the result of cooperative functioning among a number of genes collectively termed the Dorsal group. Their functioning leads to the activation of Dorsal protein, a transcription factor responsible for structuring dorsoventral polarity. The cascade of events begins with signals from the Toll receptor. These are transmitted via Tube and Pelle to Dorsal. Dorsal activation must also be accompanied by the destruction of Cactus , which otherwise would bind Dorsal, in effect imprisoning it in the cytoplasm and thereby rendering it inactive.
Tube accomplishes two critical tasks.
The first involves its interaction with Pelle, leading to the destruction of Cactus. Signal-dependent degradation of Cactus does not require the presence of Dorsal, indicating that Cactus degradation is a direct response to Pelle signaling. Tube is membrane associated in the absence of Toll. Upon activation of Toll, Tube activates Pelle, a serine-threonine kinase. Each of these interactions leads to the destruction of Cactus, which in the case of I kappa B, the vertebrate homolog of Cactus, is ultimately carried out by ubiquitin, the cell's machinery for protein degradation.