Nd. Jordan et al., S-protein mutants indicate a functional role for SBP in the self-incompatibility reaction of Papaver rhoeas, PLANT J, 20(1), 1999, pp. 119-125
The self-incompatibility response involves S-allele specific recognition be
tween stigmatic S proteins and incompatible pollen, resulting in S-specific
pollen inhibition. In Papaver rhoeas, the pollen S gene product is predict
ed to be a receptor that interacts with the stigmatic S protein in an S spe
cific manner. We recently identified an S protein binding protein (SBP) in
pollen that binds stigmatic S proteins, although apparently not in an S-all
ele-specific manner. In order to investigate the functional significance of
the interaction between S proteins and SEP, we constructed mutant derivati
ves of the S-1 protein and tested their SEP-binding activity and their biol
ogical activity. Here we present an evaluation of nine mutant derivatives o
f 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 sh
ow a concomitant reduction in their ability to inhibit incompatible pollen.
This establishes a direct link between SEP binding and inhibition of incom
patible 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.