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 D-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 carota) 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 beta-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.

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