Probable UDP-L-arabinose mutase involved in the biosynthesis of cell wall non-cellulosic polysaccharides. UDP-beta-L-arabinofuranose = UDP-beta-L-arabinopyranose Heteromers with RGP1 and RGP2. Soluble and membrane-associated. Specifically expressed in developing seeds. The conserved DXD motif is involved in enzyme activity. Reversibly glycosylated in vitro by UDP-glucose, UDP-xylose and UDP-galactose, but not UDP-mannose. Belongs to the RGP family.