Identification and partial characterization of proteins and proteoglycans encrusting the secondary cell walls of flax fibres

Four proteins were isolated from depectinised elementary fibres of flax (Linum usitatissimum L.), using either alkali or cellulase digestion treatments. All the four proteins were characterized by a deficiency or low contents of hydroxyproline and by high levels of glutamic acid/glutamine and/or aspartic acid/asparagine. The two proteoglycans solubilized with cellulase strongly reacted with beta-glucosyl Yariv reagent but not with alpha-glucosyl Yariv reagent and contained appreciable amounts of alanine, glycine, serine and threonine, suggesting a relationship with cell wall hydroxyproline deficient arabinogalactan-proteins. The two alkali-extracted proteins did not show beta-glucosyl Yariv dye. Due to the harsh treatment, they might only partially represent the original proteins. Due to its high level of glycine (41%), one of these proteins might be classified as a glycine-rich protein. The latter porypeptide, of low molecular molar mass, contained 14.6% leucine and might consist of a domain related to leucine-rich proteins. The data show that these proteins and arabinogalactan-protein-like proteoglycans were strongly associated with the secondary walls of flax fibres. Their presence in small amounts (0.1-0.4%), raises the problem of their putative structural role.