IDENTIFICATION OF RESIDUES IN A HYDROPHILIC LOOP OF THE PAPAVER-RHOEAS S-PROTEIN THAT PLAY A CRUCIAL ROLE IN RECOGNITION OF INCOMPATIBLE POLLEN

The self-incompatibility response involves S allele-specific recogniti on between stigmatic S proteins and incompatible pollen. This response results in pollen inhibition. Defining the amino acid residues within the stigmatic S proteins that participate in S allele-specific inhibi tion of incompatible pollen is essential for the elucidation of the mo lecular basis of the self-incompatibility response. We have constructe d mutant derivatives of the S-1 protein from Papaver rhoeas by using s ite-directed mutagenesis and have tested their biological activity. Th is has enabled us to identify amino acid residues in the stigmatic S p roteins of P. rhoeas that are required for S-specific inhibition of in compatible pollen. We report here the identification of several amino acid residues in the predicted hydrophilic loop 6 of the P. rhoeas sti gmatic S-1 protein that are involved in the inhibition of S-1 pollen. Mutation of the only hypervariable amino acid, which is situated in th is loop, resulted in the complete loss of ability of the S protein to inhibit S-1 pollen. This clearly demonstrates that this residue plays a crucial role in pollen recognition and may also participate in defin ing allelic specificity. We have also established the importance of hi ghly conserved amino acids adjacent to this hypervariable site. Our st udies demonstrate that both variable and conserved amino acids in the region of the S protein corresponding to surface loop 6 are key elemen ts that play a role in the recognition and inhibition of incompatible pollen in the pollen-pistil self-incompatibility reaction.