PCP-A1, A DEFENSIN-LIKE BRASSICA POLLEN COAT PROTEIN THAT BINDS THE S-LOCUS GLYCOPROTEIN, IS THE PRODUCT OF GAMETOPHYTIC GENE-EXPRESSION

Self-incompatibility (SI) in Brassica species is controlled by a singl e polymorphic locus (S) with multiple specificities. Two stigmatically expressed genes that have been cloned from this region encode the S l ocus glycoprotein (SLG) and S receptor kinase (SRK). Both appear to be essential for the operation of SI. It is believed that rejection of i ncompatible pollen grains is effected by recognition events between an as yet unidentified S locus-encoded pollen coating-borne protein and the SLG/SRK. We previously identified a small pollen coat protein PCP7 (renamed here PCP-A1, for pollen coat protein, class A, 1) that binds with high affinity to SLGs irrespective of S genotype. Here, we repor t the cloning of POP-A1 from Brassica oleracea and demonstrate that it is unlinked to the S locus. In situ localization of POP-A1 transcript s revealed that they accumulate specifically in pollen at the late bin ucleate/trinucleate stage of development rather than in the tapetum, w hich previously was taken to be the principal source of the pollen coa t. PCP-A1 is characterized by the presence of a structurally important motif consisting of eight cysteine residues shared by the plant defen sins. Based on the presence of this motif and other data, homology mod eling has been used to produce a putative structure for PCP-A1. Protei n-protein interaction analyses demonstrate that SLG exists in monomeri c and dimeric forms, both of which bind PCP-A1. Evidence is also prese nted for the existence of putative membrane-associated PCP-A1 binding proteins in stigmatic tissue.