11-35: A novel APAP1 glycoconjugate indicates a new role for proteoglycans in plant cell walls

Plant cell walls are a renewable bioenergy resource that consist of about 5% protein, 25% lignin, and 70% of an interacting carbohydrate network of cellulose, hemicellulose, and pectin. We report the detailed structural characterization of an Arabidopsis glycoconjugate, purified from Arabidopsis cell culture medium and present in plant cell walls. N-terminal sequencing, chemical, enzymatic and glycome profiling analyses revealed that the glycoconjugate contains covalently-attached protein and glycan domains previously attributed to independent wall components. Multiple apap1 T-DNA insertion mutants exhibit modified cell wall and growth properties. This glycoconjugate structure, named APAP1, has broad implications for the synthesis, structure and function of wall components. It also suggests wall proteoglycans are serving as a cross-linker that hold different polysaccharides, especially hemicelluloses and pectins, in the walls. The APAP1 structure supports the hypothesis that plant cell walls contain cross-linked polysaccharide and protein networks and provides structural information important for engineering both plants and cell wall hydrolytic enzymes to overcome biomass recalcitrance.   Funding was from National Science Foundation NSF-MCB 0646109, DOE center grant DOE DE-FG02-09ER20097 and BioEnergy Science Center grant DE-PS02-06ER64304. The BioEnergy Science Center is a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science.