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

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.