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Structure-function relationships of cell wall proteins having domains interacting with cell wall components

Besides sytematic proteomic approaches, the team is interested in elucidating the structure and the function of some cell wall proteins. During the last years, Arabidopsis thaliana DUF642 proteins and AGP31 (ArabinoGalactan Protein 31) encoded by At1g28290, have been particularly studied. These proteins have been identified as abundant proteins in cell walls of etiolated hypocotyls which undergo an important elongation within a few days.

The DUF642 (Domain of Unknown function 642, PF04862, protein family comprises 10 members in A. thaliana and is well represented in all the plant cell wall proteomes studied so far (WallProtDB). These proteins comprise a N-terminal signal peptide and two sub-domains sharing conserved amino acids predicted to be folded as beta-sheets. A few members of the family contain a GPI anchor. Such proteins have been found in all the spermatophyte genomes analyzed (Vasquez-Lobo, Roujol et al. 2012). We could show that one DUF642 protein (At3g08030) interact with cellulose in vitro (Vasquez-Lobo, Roujol et al. 2012).

AGP31 comprise a N-terminal peptide signal, a short AGP domain, a His-rich domain, a central Pro-rich central domain as well as a C-terminal domain called PAC (PRP-AGP-copntaining Cys) domain. This multi-domain organization is unique in A. thaliana and has retained our attention.

The combination of several mass spectrometry techniques associated to biochemical characterization of the protein has allowed to show the presence of Hyp residues and of Ara/Gal O-glycans in the Pro-rich domain (Hijazi et al. 2012). Several isoforms of AGP31 have been found, differing in the type and size of O-glycans or in the size of the polypeptide moiety. In particular, the AGP domain carries arabinogalactan (AG) chains which structures remain to be elucidated. Besides, in vitro interactions have been demonstrated between the PAC domain of AGP31 and galactans also found as lateral chains of type I rhamnogalacturonans (RG I) (Hijazi et al. 2014). In addition, AGP31 can interact with itself probably through the O-glycans of its Pro-rich domain. Finally, the O-glycans of the Pro-rich domain are recognized by a peanut agglutinin (PNA), suggesting possible interactions with cel wall lectins.

Altogether, these data allowed to propose a model of non covalent interactions putting AGP31 in the middle of a complex supremolecular scaffold, thus suggesting a role of cell wall organizer for AGP31. These results bring new ideas about the architecture of plant cell walls and enlighten the central role of O-glycoproteins in cell wall structure (Hijazi et al. 2014).