Structural analysis of the AGPs(arabinogalactan proteins)-pectin complex from pine coniferous greens (Pinus sylvestris).

The objective of study was to investigate potential options for the organisation of the interaction between the carbohydrate component of AGPs and pectin-containing polysaccharides from woody greens of P. sylvéstris. Pectin-specific exo- and endo-1,4-α-D-polygalacturonase was utilised to characterise a pectin-containing polymer composed of glycosyl residues, including homogalacturonan (HG), rhamnogalacturonan I (RG-I) and arabinogalactan proteins (AGPs). Enzymatic cleavage of some of the resulting polysaccharides resulted in complete removal of HG. It was determined that the released polymers were predominantly composed of RG-I attached to AGPs, forming a RG-I-AGPs complex with AGPs as the predominant component. It was demonstrated that RG-I-AGPs was the predominant polymer present, excluding minor components such as galactoglucomannan and glucuronoxylan, and had a relatively small size, i.e. 14-54 kDa. The present study provides compelling evidence for the association of RG-I with AGPs, as demonstrated by the release of HG glycans from RG-I-AGPs in the presence of pectin-specific polygalacturonase. The aforementioned observation, when considered in conjunction with the complete absence of the 2,4-α-L-Rhap residue in the RG-I core, allows the conclusion to be drawn that the carbohydrate motif of AGPs is not attached to the aforementioned residue and that RG-I proper is firmly, probably covalently, attached to AGPs.