Proteins are ubiquitous components of plant cell walls where they may account for as much as 10% of the dry weight of the wall. The wall proteins include enzymes, wall loos­ening proteins (expansins) (71), and signaling molecules (AGPs) (70) but the structural proteins are quantitatively the most important. These include the glycine-rich proteins (GRP) (72, 73), the hydroxyproline-rich glycoproteins (HRGP, extensins), the proline — and hydroxyproline-rich proteins (PRP), and non-extensin proteins (70, 74-77). The HRGP are glycoproteins bearing short arabinofuranoside side chains at the Hyp residues (70). They are highly elongated molecules with extended polyproline II helix conformations imparted by the runs of Hyp residues. The HRGP, PRP, and AGP families also contain members that are chimeras having, for example, an HRGP domain fused to a domain with AGP characteristics (70). The structural proteins of the walls are thought to form an independent wall matrix phase.