Arabinogalactan proteins (ACPs) are secreted or membraneassociated glycoproteins that have been operationally defined as binding to ,9-glucosyl Yariv artificial antigen, being rich in arabinose and galactose, and containing high levels of alanine, serine, and hydroxyproline. Using an anti-ACP monoclonal antibody (MAC 207) bound to cyanogen bromide-activated Sepharose 46, we have purified by immunoaffinity chromatography an extracellular ACP from the culture medium of suspension-cultured cells of carrot (Daucus carota). The apparent molecular mass of this highly glycosylated proteoglycan is 70 to 100 kD as judged by sodium dodecyl sulfate-polyacrylamide gels. Although its sugar analysis, j3-glucosyl Yariv binding, and high alanine, serine, and proline content are consistent with it being an AGP, the amino acid composition unexpectedly revealed this molecule to have no detectable hydroxyproline. This suggests that this glycoprotein is not a "classical" ACP, but represents the first example of a new class of hydroxyproline-poor ACPs. Deglycosylation of the ACP with anhydrous hydrogen fluoride revealed that the purified proteoglycan contains probably a single core protein with an apparent molecular mass of 30 kD. Direct visualization of the native ACP in the electron microscope showed ellipsoidal putative ACP monomers, approximately 25 nm by 15 nm, that showed a strong tendency to self assemble into higher-order structures. Upon desiccation, the glycosylated ACP formed paracrystalline arrays visible in the light microscope. Polarized Fourier transform infrared microspectroscopy of these arrays demonstrated a high degree of polarization of the sugar moieties under these conditions. These results put possible constraints on current models of ACP structure; a putative role for these nove1 AGPs as pectin-binding proteins is discussed.
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