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 inin vitro synthesized dimer. Pb²⁺, La³⁺, and Ca²⁺ increase dimer formationin 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.