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PLANT PHYSIOLOGY , Vol 103, Issue 1 115-123, Copyright © 1993 by American Society of Plant Biologists
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CELL BIOLOGY AND SIGNAL TRANSDUCTION |
A Novel Hydroxyproline-Deficient Arabinogalactan Protein Secreted by Suspension-Cultured Cells of Daucus carota (Purification and Partial Characterization)
T. C. Baldwin, M. C. McCann and K. Roberts
Department of Cell Biology, John Innes Institute, Colney Lane, Norwich, NR4 7UH, United Kingdom
Arabinogalactan proteins (AGPs) are secreted or membrane-associated
glycoproteins that have been operationally defined as binding to
[beta]-glucosyl Yariv artificial antigen, being rich in arabinose and
galactose, and containing high levels of alanine, serine, and
hydroxyproline. Using an anti-AGP monoclonal antibody (MAC 207) bound to
cyanogen bromide-activated Sepharose 4B, we have purified by immunoaffinity
chromatography an extracellular AGP 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,
[beta]-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" AGP, but
represents the first example of a new class of hydroxyproline-poor AGPs.
Deglycosylation of the AGP 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 AGP in
the electron microscope showed ellipsoidal putative AGP monomers,
approximately 25 nm by 15 nm, that showed a strong tendency to self
assemble into higher-order structures. Upon desiccation, the glycosylated
AGP 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 AGP structure; a putative role for these novel AGPs as pectin-binding
proteins is discussed.
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