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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.