Glycine-rich proteins as structural components of plant cell walls

Glycine-rich proteins (GR-Ps) have been found in the cell walls of many hig her plants and form a third group of structural protein components of the w all in addition to extensins and proline-rich proteins. The primary sequenc es of GRPs contain more than 60% glycine. GRPs are localized mainly in the vascular tissue of the plant, and their coding genes provide an excellent s ystem to analyze the molecular basis of vascular-specific gene expression. In French bean, the major cell wall GRP has been localized at the ultrastru ctural level in the modified primary cell wall of protoxylem. Immunological studies showed that it forms a major part of these highly extensible and s pecialized cell walls. Specific digestion of GRP 1.8 from bean by collagena se suggests that it shares structural similarities with collagen. The prote in is synthesized by living protoxylem cells as well as xylem parenchyma ce lls. After cell death, GRPs are exported from neighboring xylem parenchyma cells to the protoxylem, wall, a rare example of protein transport between cells in plants. We. propose that GR-Ps are part of a repair system of the plant during the stretching phase of protoxylem.