IDENTIFICATION OF A MEMBRANE GLYCOPROTEIN IN POLLEN OF PAPAVER-RHOEASWHICH BINDS STIGMATIC SELF-INCOMPATIBILITY (S-) PROTEINS

Recombinant stigmatic self-incompatibility (S-) proteins from Papaver rhoeas have previously been shown to be biologically functional, inhib iting only pollen of the same S-genotype. In an attempt to identify mo lecules in pollen which interact with these proteins, Western ligand b lotting was used, with the recombinant S-proteins as probes followed b y immunodetection of the bound S-protein. This revealed that pollen of all S-genotypes tested contained a 70-120 kDa protein which bound the S-1, S-3 and S-8 proteins in an indistinguishable manner. Binding was destroyed by pretreatment of blots with periodate, implicating a glyc oprotein with activity being dependent on the glycan moiety. The activ ity completely partitioned into the detergent phase on condensation wi th Triton X-114, indicating an integral membrane protein. On aqueous t wo-phase partition of microsomal membrane preparations, the majority o f the binding activity partitioned into the upper phase, suggesting th at the molecule is located in the plasma membrane. No equivalent bindi ng could be detected in extracts of leaves, stems, roots or stigmas of P. rhoeas, nor in immature anthers. Identical activity was detected i n pollen of some other Papaver species, but not in pollen of Brassica oleracea, Nicotiana tabacum or Petunia hybrida. The presence in mature pollen of P. rhoeas of a plasma membrane glycoprotein which binds S-p roteins from the stigma of the same species, albeit in a non S-allele- specific manner, strongly suggests that this molecule has a role somew here in the interaction of the stigma proteins with pollen. The activi ty is not that expected of an S-specific receptor, but by analogy with certain mammalian systems the molecule may act as an accessory recept or, or co-receptor, the presence of which may be essential for a funct ional interaction with an S-specific receptor.