IMMUNOCHEMICAL COMPARISON OF MEMBRANE-ASSOCIATED AND SECRETED ARABINOGALACTAN-PROTEINS IN RICE AND CARROT

Arabinogalactan-proteins (AGPs) occurring in suspension-cultured rice (Oryza sativa L.) cells, their conditioned medium and at the rice root apex were investigated using monoclonal antibodies and the AGP-bindin g beta-glucosyl Yariv reagent (beta GlcY). A monoclonal antibody, LM2, was generated that recognized an acidic carbohydrate epitope common t o two soluble AGPs occurring in the conditioned medium of proliferatin g rice cells, membrane-associated AGPs (rmAGP) in the cultured cells a nd two AGPs at the rice root apex. In addition, LM2 recognized AGPs se creted by suspension-cultured carrot (Daucus carota L.) cells. The two AGPs of the rice culture medium, srAGP1 and srAGP2, were discriminate d by their mobilities during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, reaction with beta GlcY, the presence of arabinogala ctan epitopes and anion-exchange chromatography. The association of rm AGP with the plasma membrane was investigated by Triton-X-114/aqueous partitioning of both microsomal and plasma-membrane preparations and r mAGP was found to partition into the detergent phase, indicating that AGPs are hydrophobic plasma-membrane proteins in rice. This was in con trast to plasma-membrane AGPs of suspension-cultured carrot cells that partitioned into the aqueous phase. At the rice root apex most of the AGP was associated with the microsomal fraction and also partitioned into the detergent phase, although a distinct high-molecular-mass AGP entered the aqueous phase.