Hydrophobic interactions of the structural protein GRP1.8 in the cell wallof protoxylem elements

The glycine-rich structural protein GRP1.8 of French bean (Phaseolus vulgar is) is specifically localized in the modified primary cell walls of protoxy lem elements. Continuous deposition of GRP1.8 into the cell walls during el ongation growth of the plant suggests that GRP1.8 is part of a repair mecha nism to strengthen the protoxylem. In this work, a reporter-protein system was developed to study the interaction of GRP1.8 with the extracellular mat rix. Fusion proteins of a highly soluble chitinase with different domains o f GRP1.8 were expressed in the vascular tissue of tobacco (Nicotiana tabacu m), and the chemical nature of the interaction of these fusion proteins in the cell wall compartment was analyzed. In contrast with chitinase that req uired only low-salt conditions for complete extraction, the different chiti nase/GRP1.8 fusion proteins were completely extracted only by a nonionic or ionic detergent, indicating hydrophobic interactions of GRP1.8. The same i nteractions were found with the endogenous GRP1.8 in bean hypocotyls. Ln ad dition, in vitro experiments indicate that oxidative cross-linking of tyros ines might account for the insolubilization of GRP1.8 observed in later sta ges of protoxylem development. Our data suggest that GRP1.8 forms a hydroph obic protein-layer in the cell wall of protoxylem vessels.