Involved in cell size determination. May serve as a chaperone for expansins through the secretory pathway. Expressed in flowers and stems. Detected in trichomes, guard cells, root vascular tissue, root hairs, pollen sacs, sepals and styles of pistils. Barely detectable in 6 days after-germination (DAG) seedlings, but highly expressed in 14 DAG seedlings. The BURP domain located at the C-terminus has not been identified in non-plant proteins (PubMed:9790599). It is critical for PGL3's role in cell growth (PubMed:26106400). Slightly reduced size of the plant. Atpgl1, atpgl2 and atpgl3 triple mutants produce smaller leaves and petioles. Unlike the tomato GP1, the BURP domain of AtPGL3 is not cleaved when the protein is secreted to the cell wall.