Pectin Rhamnogalacturonan II: On the "Small Stem with Four Branches" in the Primary Cell Walls of Plants

Rhamnogalacturonan II (RG-II) is a type of block copolymer of complex pectins that represents a quantitatively minor component of the primary cell walls of land (vascular) plants. The structural composition of RG-II is almost totally sequenced and appears to be remarkably conserved in all tracheophytes so far examined. The backbone of RG-II, released from complex (cell wall) pectins by endo-polygalacturonase (Endo-PG) treatment, has been found to contain up to 15 (1→4)-linked-α-D-GalpA units, some of which carry four well-defined side chains, often referred to as A-, B-, C-, and D-side chains. Nevertheless, the relative locations on the backbone of these four branches, especially the A chain, remain to be ascertained. A combination of different data suggests that neither the terminal nonreducing GalA nor the contiguous GalA unit is likely to be the branching point of the A chain, but probably the ninth GalA residue from the reducing end, assuming a minimum backbone length of 11 (1→4)-linked-α-d-GalpA. The latest reports on RG-II are here highlighted, with a provided update for the macrostructure and array of functionalities.