Repetitive proline‐rich proteins in the extracellular matrix of the plant cell

Several classes of hydroxyproline‐rich proteins have been found in the cell walls of plants. Monomeric forms of the proteins can be solubilized from the walls, but the majority of the proteins are insolubilized by as yet unidentified crosslinks. The proteins have repeated sequences and are often rich in basic amino acids. The repeat proline‐rich region may be serving to generate an elongated rod‐like structure while the regularly spaced lysines probably interact with the acidic pectins. Several of the genes coding for these proteins have been isolated, and their expression has been found in some cases to be tissue specific and in others inducible by hormones and various types of stress.