The plant cell wall polysaccharide rhamnogalacturonan II self-assembles into a covalently cross-linked dimer.
The location of the 1:2 borate-diol ester cross-link in the dimer of the plant cell wall polysaccharide rhamnogalacturonan II (RG-II) has been determined. The ester cross-links the apiofuranosyl residue of the 2-O-methyl-D-xylose-containing side chains in each of the subunits of the dimer. The apiofuranosyl residue in each of the two aceric acid-containing side chains is not esterified. The site of borate esterification is identical in naturally occurring and in in vitro synthesized dimer. Pb2+, La3+, and Ca2+ increase dimer formation in vitro in a concentration- and pH-dependent manner. Pb2+ is the most effective cation. The dimer accounts for 55% of the RG-II when the monomer (0.5 mM) is treated for 5 min at pH 3.5 with boric acid (1 mM) and Pb2+ (0.5 mM); at pH 5 the rate of conversion is somewhat slower. Hg2+ does not increase the rate of dimer formation. A cation's charge density and its ability to form a coordination complex with RG-II, in addition to steric factors, may regulate the rate and stability of dimer formation in vitro. Our data provide evidence that the structure of RG-II itself determines which apiofuranosyl residues are esterified with borate and that in the presence of boric acid and certain cations, two RG-II monomers self-assemble to form a dimer.
Ishii
T
T
Forestry and Forest Products Research Institute, Tsukuba Norin Kenkyu, Danchi-Nai, Ibaraki 305-8687, Japan.
Matsunaga
T
T
Pellerin
P
P
O'Neill
M A
MA
Darvill
A
A
Albersheim
P
P
D001888:Boric Acids / Q000737:chemistry; D002240:Carbohydrate Sequence; D002473:Cell Wall / Q000737:chemistry*; D019281:Dimerization; D008401:Gas Chromatography-Mass Spectrometry; D007700:Kinetics; D008969:Molecular Sequence Data; D010368:Pectins / Q000737:chemistry*; D010944:Plants / Q000737:chemistry*
D001888:Boric Acids; C042492:rhamnogalacturonan II; D010368:Pectins; C032688:boric acid