Figure 5.

Two models for the molecular mechanism of signal transduction via SRK in the SI response. (A) This model proposes that SRK (open rectangle) spontaneously associates as a dimer in the plasmalemma of stigmatic papillar cells before pollination. Interaction with a self-pollen borne ligand (filled circle) induces a conformational change of SRK, which allows the recruitment of cytoplasmic targets (large hatched circles) that mediate the SI response. In the example shown here, the cytoplasmic targets recognize phosphorylated residues (small circles) on the SRK protein. (B) In the second model, SRK is also present as a constitutively formed dimer but the activation of SI response by pollen-ligand (in black) requires both binding to SRK in an allele-specific manner, and then binding to a second as-yet-unknown coreceptor (hatched rectangle), which does not need to exhibit any allelic specificity. Signal transduction then may involve interaction of the second receptor with cytosolic targets analogous to those described for serine/threonine receptor kinases in animals.