A Family of Abundant Plasma Membrane-associated Glycoproteins Related to the Arabinogalactan Proteins Is Unique to Flowering Plants

We have identified a family of abundant peripheral plasma membrane glycoproteins that is unique to flowering plants. They are identified by a monoclonal antibody, MAC 207, that recognizes an epitope containing L-arabinose and o-glucuronic acid. Immunofluorescence and immunogold labeling studies locate the MAC 207 epitope to the outer surface of the plasma membrane both in protoplasts and in intact tissues. In some cells MAC 207 also binds to the vacuolar membrane, probably reflecting the movement of the plasma membrane glycoproteins in the endocytic pathway. The epitope recognized by MAC 207 is also present on a distinct soluble proteoglycan secreted into the growth medium by carrot (Daucus carom) suspension culture cells. Biochemical evidence identifies this neutral proteoglycan as a member of the large class of arabinogalactan proteins (AGPs), and suggests a structural relationship between it and the plasma membrane glycoproteins. AGPs have the property of binding to /5-glycans, and we therefore propose that one function of the AGP-related, plasma membrane-associated glycoproteins may be to act as cell surface attachment sites for cell wall matrix polysaccharides. T HE presence of a rigid cell wall makes the plant plasma membrane a relatively inaccessible structure. Quantitative data on protein composition (Kjellbom and Larsson, 1984) and protein topography (Grimes and Breidenbach, 1987) are beginning to facilitate comparative studies on plasma membranes, but knowledge of constitutive cell surface glycoproteins is still fragmentary. Information of the kind available for animal cell surface glycoproteins (Hynes, 1985) and proteoglycans (Hook et al., 1984) is entirely lacking. Agglutination and fluorescence labeling of plant protoplasts by lectins has permitted the identification of terminal glycan residues on the outer face of the plasma membrane (Walko et al., 1987), presumably components of glycoproteins or glycolipids. The presence among these of members of the class of glycoproteins termed arabinogalactan proteins (AGPs)' (Fincher et al., 1983) has been surmised from protoplast agglutination with the/3-glycosyl Yariv reagents (Larkin, 1977, 1978; Samson et al., 1983), and from the coincident migration during isoelectric focussing of hydroxyproline-rich membrane components and components with affinity for ~-galactosyl Yariv reagent (Samson et al., 1983). However, since Yariv reagents do not interact with AGPs specifically (Jermyn, 1978), and hydroxyproline is the principal amino acid of other arabinosylated plant cell extracellular matrix glycoproteins such as extensin (Showalter and 1. Abbreviation used in this paper: AGE arabinogalactan protein. Varner, 1989), it has not yet been possible to confirm that some AGPs are plasma membrane components. Moreover, few of the antisera and monoclonal antibodies that recognize flowering plant cell plasma membranes (Norman et al., 1986; Villaneuva et al., 1986; Grimes and Breidenbach, 1987; Lynes et al., 1987; Bradley et al., 1988; Meyer et al., 1988) have been biochemically characterized, and at present only identification of the plasma membrane H÷-ATPase has been achieved immunologically (DuPont et al., 1988). In this report we describe the characterization of a monocional antibody (MAC 207), originally prepared from immunizations with peribacteroid membrane of pea root nodules (Bradley et al., 1988) that recognizes a family of antigens associated with the plant cell plasma membrane. We show that MAC 207 also binds to an AGP secreted by carrot suspension culture cells and reason that the composition of the epitope and the structural specificity of the antibody can be used to define a specific family of AGP-related glycoproteins that are associated with the extracellular face of the plasma membrane. We suggest that this family of glycoproteins are plasma membrane components in somatic cells of many or all flowering plants, and that they may function as cell surface receptors for cell wall matrix molecules. Materials and Methods