A PLANT-SURFACE PROTEIN SHARING STRUCTURAL-PROPERTIES WITH ANIMAL INTEGRINS

Using a polyclonal antibody (P23) generated against the human platelet integrin alpha IIb beta 3 and a FITC-conjugate secondary antibody, fl uorescence is observed at the surface of protoplasts isolated from Ara bidopsis thaliana and Rubus fruticosus. Arabidopsis thaliana cells gro wn in suspension culture containing P23 and glycylarginylglycylasparcy lserine (GRGDS), a synthetic peptide containing the RGD sequence found in many extracellular matrix adhesive proteins demonstrated aberrant cell wall/plasma membrane interactions and organization. When glycopro teins from these plants, purified on a concanavalin A Sepharose 4B, we re subjected to SDS/PAGE and Western blotting, under reduced and non-r educed conditions, immunoblots probed with P23 revealed bands in both species. A shift in electrophoretic mobility is observed to different apparent molecular mass when no reducing agent is present. When purifi ed by immunoaffinity chromatography on anti-alpha IIb beta 3 Sepharose or Sepharose linked to the synthetic peptide D-Arg-Gly-Asp-Trp, the m ajor antigenic components detected migrate at 30 kDa and 60 kDa in the first experiment and 60 kDa in the second one. Only the 60-kDa compon ent is immunodetected with antibodies specific for either the beta 3 p latelet chain or the alpha IIb polypeptide, suggesting the presence of two polypeptides co-migrating. To address more precisely the structur e of this complex in plants, competition assays were performed. A sign ificant inhibition is observed with CS3 a monoclonal antibody that int eracts with the complexed form alpha IIb beta 3 but nut the dissociate d subunits. Further structural similarities with the animal alpha IIb beta 3 complex is demonstrated with Western blotting detection after p lant glycoproteins immunoprecipitation with CS3 in absence or presence of 5 mM EDTA to dissociate the complex. We also present data on the c haracterization of a polyclonal antibody, named AcAt2, raised against Arabidopsis glycoco-proteins purified by affinity chromatography on a D-RGDW column and eluted with the same peptide, that specifically inte racts with the animal alpha IIb beta 3 receptor.