Immunochemical comparison of membrane-associated and secreted arabinogalactan-proteins in rice and carrot
Smallwood, M. | Yates, E.A. | Willats, W.G.T. | Martin, H. | Knox, J.P.
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-binding beta-glucosyl Yariv reagent (beta GlcY). A monoclonal antibody, LM2, was generated that recognized an acidic carbohydrate epitope common to two soluble AGPs occurring in the conditioned medium of proliferating rice cells, membrane-associated AGPs (rmAGP) in the cultured cells and two AGPs at the rice root apex. In addition, LM2 recognized AGPs secreted by suspension-cultured carrot (Daucus carota L.) cells. The two AGPs of the rice culture medium, srAGP1 and srAGP2, were discriminated by their mobilities during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, reaction with beta GlcY, the presence of arabinogalactan epitopes and anion-exchange chromatography. The association of rmAGP with the plasma membrane was investigated by Triton-X-114/aqueous partitioning of both microsomal and plasma-membrane preparations and rmAGP was found to partition into the detergent phase, indicating that AGPs are hydrophobic plasma-membrane proteins in rice. This was in contrast 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.
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