Hydroxyproline‐Rich Glycoproteins: Form and Function

The extracellular matrix is a morphogenetic substrate based on scaffold proteins comprising hydroxyproline-rich glycoproteins (HRGPs): collagens in animals and extensins in plants. Repetitive peptide motifs and glycomodules define the extensin superfamily, richly diverse in both structure and function. There are two major groups: highly basic classical extensins form insoluble covalent networks, while the classical arabinogalactan proteins (AGPs) are acidic and easily solubilized. In both extensins and AGPs the primary sequence encodes extensive post-translational modifications: addition of a GPI anchor to AGPs, crosslinking of extensins via isodityrosine (Idt) and diisodityrosine (di-Idt), and hydroxylation of proline residues where subsequent O-Hyp glycosylation defines the glycomodule. Non-contiguous Hyp directs addition of small, compact, beadlike arabinogalactan polysaccharides that coat the plasma membrane and hypothetically also act as a pectic plasticizer. However, in the basic extensins blocks of contiguous Hyp direct addition of small oligoarabinosides. The resulting hydrophilic glycomodules alternate with short hydrophobic Idt motifs to form self-assembling amphiphiles that react with acidic pectin. Thus an extensin pectate coacervate may be a template for the formation of the cell plate. Evolution of the cell plate and the acquisition of turgor required for land plant evolution were thus crucially dependent on HRGPs.