Identification of a membrane glycoprotein in pollen of Papaver rhoeas which binds stigmatic self-incompatibility (S-) proteins

Recombinant stigmatic self-incompatibility (S-) proteins from Papaver rhoeas have previously been shown to be biologically functional, inhibiting only pollen of the same S -genotype. In an attempt to identify molecules in pollen which interact with these proteins, Western ligand blotting was used, with the recombinant S-proteins as probes followed by immunodetection of the bound S-protein. This revealed that pollen of all S -genotypes tested contained a 70–120 kDa protein which bound the S₁, S₃ and S₈ proteins in an indistinguishable manner. Binding was destroyed by pretreatment of blots with periodate, implicating a glycoprotein with activity being dependent on the glycan moiety. The activity completely partitioned into the detergent phase on condensation with Triton X-114, indicating an integral membrane protein. On aqueous two-phase partition of microsomal membrane preparations, the majority of the binding activity partitioned into the upper phase, suggesting that the molecule is located in the plasma membrane.
No equivalent binding could be detected in extracts of leaves, stems, roots or stigmas of P. rhoeas , nor in immature anthers. Identical activity was detected in 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-proteins from the stigma of the same species, albeit in a non S -allele-specific manner, strongly suggests that this molecule has a role somewhere in the interaction of the stigma proteins with pollen. The activity is not that expected of an S-specific receptor, but by analogy with certain mammalian systems the molecule may act as an accessory receptor, or co-receptor, the presence of which may be essential for a functional interaction with an S-specific receptor.