S-protein mutants indicate a functional role for SBP in the self-incompatibility reaction of Papaver rhoeas

The self-incompatibility response involves S-allele specific recognition between stigmatic S proteins and incompatible pollen, resulting in S-specific pollen inhibition. In Papaver rhoeas, the pollen S gene product is predicted to be a receptor that interacts with the stigmatic S protein in an S specific manner. We recently identified an S protein binding protein (SBP) in pollen that binds stigmatic S proteins, although apparently not in an S-allele-specific manner. In order to investigate the functional significance of the interaction between S proteins and SEP, we constructed mutant derivatives of the S-1 protein and tested their SEP-binding activity and their biological activity. Here we present an evaluation of nine mutant derivatives of the S-1 protein. Western ligand blotting was used to show that mutations to amino acid residues in predicted loops 2 and 6 of the S-1 protein cause significant reductions in their SEP-binding activity. These same mutants show a concomitant reduction in their ability to inhibit incompatible pollen. This establishes a direct link between SEP binding and inhibition of incompatible pollen and implicates SEP as a pollen component playing a key role in the self-incompatibility reaction. We discuss the possible nature of the contribution of SEP in the S specific rejection of incompatible pollen.