HRGPs

Hydroxyproline-rich glycoproteins (HRGPs) are plant cell wall proteins rich in hydroxyproline and extensively O-glycosylated. They are secreted into the apoplast and contribute to wall structure. Hydroxyproline results from the enzymatic hydroxylation of proline, enabling attachment of carbohydrate chains primarily made of arabinose and galactose.

HRGPs are commonly divided into three families: extensins, arabinogalactan proteins (AGPs), and proline-rich proteins (PRPs). Extensins are small, rich in Ser-Hyp repeats and form cross-linked networks; AGPs are highly glycosylated and often GPI-anchored; PRPs are proline-rich with variable glycosylation.

Biosynthesis and localization: HRGPs are synthesized with signal peptides and directed into the secretory pathway. Proline is hydroxylated by prolyl-4-hydroxylase, then O-glycosylated in the Golgi. The mature proteins are secreted to the wall and can be cross-linked by peroxidases.

Functions: HRGPs contribute to wall architecture and mechanics, regulate cell expansion, facilitate pollen tube growth and root hair formation, and participate in defense signaling during development and stress.

Distribution and significance: HRGPs occur in virtually all land plants, with tissue- and stage-specific abundance. They are studied as components of the primary wall and as indicators of wall remodeling; understanding HRGPs informs plant extracellular matrix biology.