Two different families of hydroxyproline-rich glycoproteins in melon callus: biochemical and immunochemical studies

Abstract

Two different families of hydroxyproline-rich glycoproteins, HRGP(1) and HRGP(2), have been isolated from melon callus and separated by ion exchange chromatography on CM-sepharose. HRGP(1) corresponds to an arabinogalactan protein. The sugar portion of HRGP(1) accounts for 94% of the molecule and contains galactose (66%) and arabinose (34%); these residues are present as polysaccharide side chains attached to hydroxyproline. Hydroxyproline is the main amino acid residue (46%) of the protein moiety. The arabinogalactan protein nature of HRGP(1) has been checked by its ability to positively react with the beta-glucosyl Yariv antigen; the (3)H-labeled deglycosylated HRGP(1) also called HRP(1) migrates upon electrophoresis as a single band of molecular weight 76,000. HRGP(2) was fractionated by affinity chromatography on heparin-Ultrogel into three different glycoproteins, HRGP(2a,2b) and (2c). Two of these glycoproteins behave as polycations (HRGP(2b) and (2c)) and are chemically distinct from HRGP(2a). HRGP(2b) is the most abundant component and contains 41% protein and 50% sugar. Hydroxyproline, lysine, tyrosine, and arabinose are the most prominent residues of their respective moiety. The glycosylation pattern of hydroxyproline indicates that HRGP(2b) is related to and possibly a precursor of the wall HRGP; as in melon cell wall HRGP, Hyp-Ara(3) predominates, and small amounts of a putative Hyp-Ara(5) a hitherto unreported hyp-arabinoside, are recorded. The molecular weight of HRP(2b), the protein portion of HRGP(2b) is 55,000 +/- 5,000, as estimated after deglycosylation of the molecule with trifluoromethane sulfonic acid. Antibodies have been raised against HRGP(2b) and HRP(2b). Immunodiffusion shows that each antigen (HRGP(2b) or HRP(2b)) reacts with its own IgG, and cross-reacts with the heterologous IgG, thereby indicating the presence of common (unglycosylated) and specific (glycosylated and deglycosylated) epitopes. The arabinogalactan protein HRGP(1) is not recognized by either antibody and HRGP(2b) does not react with the Yariv antigen. Immunoprecipitation of (3)H-labeled HRP(1) and HRP(2b) in the presence of goat antirabbit IgG, followed by gel electrophoresis, allows to recover HRP(2b) only. Again, HRP(2b) is immunoprecipitated by the two antisera.