UDP-L-arabinose mutase involved in the biosynthesis of cell wall non-cellulosic polysaccharides. Catalyzes the interconvertion of UDP-L-arabinopyranose (UDP-Arap) and UDP-L-arabinofuranose (UDP-Araf). Preferentially catalyzes the formation of UDP-Arap from UDP-Araf. At thermodynamic equilibrium in vitro the ratio of the pyranose form over the furanose form is 95:5. Is not active on other UDP-sugars (UDP-Gal, UDP-Xyl, UDP-Glc, GDP-Man and GDP-Fuc). Is probably active as heteromer in vivo. UDP-beta-L-arabinofuranose = UDP-beta-L-arabinopyranose Heterodimer with RGP1. 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. Truncated N-terminus.