Rhamnogalacturonan-II, a pectic polysaccharide in the walls of growing plant cell, forms a dimer that is covalently cross-linked by a borate ester. In vitro conditions for the formation and hydrolysis of the dimer.
Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide present in the walls of growing plant cells. We now report that RG-II, released by endopolygalacturonase treatment of the walls of suspension-cultured sycamore cells and etiolated pea stems, exists mainly as a dimer that is cross-linked by a borate ester. The borate ester is completely hydrolyzed at room temperature within 30 min at pH 1, partially hydrolyzed between pH 2 and 4, and stable above pH 4. The dimer is formed in vitro between pH 2.4 and 6. 2 by treating monomeric RG-II (0.5 mM) with boric acid (1.2 mM); the dimer formed after 24 h at pH 3.4 and 5.0 accounts for approximately 30 and approximately 5%, respectively, of the RG-II. In contrast, the dimer accounts for approximately 80 and approximately 54% of the RG-II when the monomer is treated for 24 h at pH 3.4 and 5.0, respectively, with boric acid and 0.5 m Sr2+, Pb2+, or Ba2+. The amount of dimer formed at pH 3.4 or 5.0 is not increased by addition of 0.5 mM Ca2+, Cd2+, Cu2+, Mg2+, Ni2+, and Zn2+. Steric considerations appear to regulate dimer formation since those divalent cations that enhance dimer formation have an ionic radius >1.1 A. Our data suggest that the borate ester is located on C-2 and C-3 of two of the four 3'-linked apiosyl residues of dimeric RG-II. Our results, taken together with the results of two previous studies (Kobayashi, M., Matoh, T., and Azuma, J.-I. (1996) Plant Physiol. 110, 1017-1020; Ishii, T., and Matsunaga, T. (1996) Carbohydr. Res. 284, 1-9) provide substantial evidence that this plant cell wall pectic polysaccharide is covalently cross-linked.
O'Neill
M A
MA
Complex Carbohydrate Research Center and Department of Biochemistry and Molecular Biology, The University of Georgia, Athens, Georgia 30602-4712, USA.
Warrenfeltz
D
D
Kates
K
K
Pellerin
P
P
Doco
T
T
Darvill
A G
AG
Albersheim
P
P
D001464:Barium / Q000378:metabolism; D001881:Borates / Q000737:chemistry*; D002413:Cations, Divalent / Q000494:pharmacology; D002473:Cell Wall / Q000737:chemistry; D003432:Cross-Linking Reagents; D019281:Dimerization; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D066298:In Vitro Techniques; D007854:Lead / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D018532:Pisum sativum; D010368:Pectins / Q000737:chemistry*; D010944:Plants; D011487:Protein Conformation; D013324:Strontium / Q000378:metabolism; D014920:Wine
D001881:Borates; D002413:Cations, Divalent; D003432:Cross-Linking Reagents; C042492:rhamnogalacturonan II; D001464:Barium; D007854:Lead; D010368:Pectins; D013324:Strontium