Secondary structure and post-translational modifications of the leucine-rich repeat protein PGIP (polygalacturonase-inhibiting protein) from Phaseolus vulgaris

A detailed analysis of the secondary structure has been carried out on the polygalacturonase-inhibiting protein (PGIP) from Phaseolus, vulgaris, a leu cine-rich repeat (LRR) protein present in the cell wall of many plants. Far -UV CD and infrared spectroscopies coupled to constrained secondary structu re prediction methods indicated the presence of 12 alpha- and 12 beta -segm ents, thus allowing a schematic representation of three domains of the prot ein, namely, the central LRR region and the two cysteine-rich flanking doma ins. Peptides from endoproteinase-degraded PGIP were analyzed by mass spect rometry, and four disulfide bonds were identified. Mass spectrometric analy sis in combination with glycosidase treatments revealed two N-linked oligos accharides located on Asn 64 and Asn 141. The main structure resembled the typical complex plant N-glycan consisting of a core pentasaccharide beta1,2 -xylosylated, carrying an alpha1,3-fucose linked to the innermost N-acetylg lucosamine and one outer arm N-acetylglucosamine residue, The schematic rep resentation of PGIP structural domains is discussed in the framework of the structure and function of LRR proteins.