Fig. 5. Model of signal transduction pathways during terminal differentiation. When GABA is bound to GrlE on the surface of prespore cells, the signal is transduced via PI3 kinase and PkbR1 to result in release of AcbA, the precursor of SDF-2. The same pathway in prestalk cells results in the exposure of the protease domain of TagC, such that it can process AcbA into SDF-2. When glutamate is bound to GrlE, a distinct signal transduction pathway results in the inhibition of release of AcbA in response to SDF-2. When SDF-2 binds its receptor DhkA, phospho-relay to RdeA is inhibited and may be reversed, resulting in a decrease in the activity of the internal cAMP phosphodiesterase RegA. The subsequent increase in cAMP generated by the adenylyl cyclase ACR activates PKA which triggers release of AcbA, as well as rapid encapsulation of prespore cells. Inhibition of phosphorelay to RegA in prestalk cells results in exposure of the protease domain of TagC.