The Plant Cell Wall Polysaccharide Rhamnogalacturonan II Self-assembles into a Covalently Cross-linked Dimer

Abstract

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-Omethyl-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. Pb 2؉ , La 3؉ , and Ca 2؉ increase dimer formation in vitro in a concentration-and pH-dependent manner. Pb 2؉ 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 Pb 2؉ (0.5 mM); at pH 5 the rate of conversion is somewhat slower. Hg 2؉ 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 selfassemble to form a dimer.Rhamnogalacturonan II (RG-II) 1 is a low molecular weight, structurally complex pectic polysaccharide released from the primary walls of plants by treatment with endo-␣-1,4-polygalacturonase (1). The results of recent studies have established that RG-II exists predominantly in the cell walls of all higher plants as a dimer (dRG-II-B) that is cross-linked by a 1:2 borate-diol ester (2-5). Borate ester cross-linking of pectin may be required for the normal growth and development of plants (4 -6) because boron deficiency results in the inhibition of plant growth and in the formation of cell walls with markedly altered physical properties (7-11).A single 1:2 borate-diol ester is believed to cross-link two RG-II molecules. 1 mol of the dimer contains 1 mol of boron (3,4,(12)(13)(14). We have proposed that two of the four 3Ј-linked apiofuranosyl (Apif) residues present in dRG-II-B are crosslinked by borate because the dimer contains approximately equimolar amounts of 3Ј-and 2,3,3Ј-linked Apif residues, whereas monomeric RG-II (mRG-II) contains only 3Ј-linked Apif residues (4, 12). However, two different Apif-containing side chains are attached to the backbone of each RG-II monomer (chains A and B in Fig. 1). It is not known whether the borate cross-link involves the Apif residue in one of each of the two types of Apif-containing side chains or if the borate is attached to the same type of Apif-containing side chains in each monomer.The ability to form dRG-II-B from mRG-II and boric acid in vitro provides a convenient model system to examine the mechanism of dimer formation (4). In addition, such studies are likely to provide information on the ability of dRG-II-B, which is present in fermented beverages such as wine, to form complexes with heavy me...

Occurrence of the Primary Cell Wall Polysaccharide Rhamnogalacturonan II in Pteridophytes, Lycophytes, and Bryophytes. Implications for the Evolution of Vascular Plants

“…However, these amounts do not necessarily reflect the amount of RG-II dimer present in the walls because boron may be bound non-specifically to polysaccharides. The (O'Neill et al, 1996;Ishii et al, 1999). Signals corresponding to these elements co-eluted with 11 B in the Driselase digests of all of the pteridophyte, lycophyte, and bryophyte walls (data not shown), which provides additional evidence that all of these walls contain RG-II.…”

Functional Characterization of UDP-apiose Synthases from Bryophytes and Green Algae Provides Insight into the Appearance of Apiose-containing Glycans during Plant Evolution

“…The apiosyl residues of side chain A in two RG-II molecules are cross-linked by a borate diester to form the RG-II dimer (7)(8)(9)(10). At least 90% of the RG-II in primary walls exists as a dimer (11), and a reduction in the extent of RG-II cross-linking typically results in the formation of abnormal cell walls (12). Plants carrying mutations that affect Api metabolism as well as RG-II structure and cross-linking are dwarfed or fail to develop normally (13)(14)(15)(16).…”

The Gene Expression and Enzyme Activity of Plant 3-Deoxy-D-Manno-2-Octulosonic Acid-8-Phosphate Synthase Are Preferentially Associated with Cell Division in a Cell Cycle-Dependent Manner

“…Despite a complex structure, RG-II is evolutionarily conserved in the plant kingdom because it is present in the primary cell wall of all higher plants predominantly in the form of a dimer that is crosslinked by a borate di-ester (dRG-II-B) between two apiosyl residues (O'Neill et al, 1996;Ishii et al, 1999). Nevertheless the precise function of RG-II remains to be fully established.…”

“…B deficiency results in altered plant growth and changes in cell wall architecture (Fleischer et al, 1998(Fleischer et al, , 1999Ishii et al, 2001). Thus, B-mediated cross-linking of RG-II generates a covalently crosslinked pectic network that is involved in the regulation of cell wall properties and plant growth (Fleischer et al, 1999;Ishii et al, 1999Ishii et al, , 2001.…”