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) in vitro. 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) (PubMed:21478444). Functions redundantly with RGP2 and is essential for proper cell walls and pollen development. Probably involved in the formation of the pectocellulosic cell wall layer intine. Is probably active as heteromer in vivo (PubMed:17071651). UDP-beta-L-arabinofuranose = UDP-beta-L-arabinopyranose Heteromers with RGP1, RGP4 and RGP5. Soluble and membrane-associated. Predominantly expressed in shoot and root apical meristems. Expressed in epidermal cells of leaves, inflorescence stems and seed coat. Expressed in pollen. 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. No visible phenotype under normal growth condition, but significant reduction in total cell wall arabinose. Rgp1 and rgp2 double mutant is male gametophyte lethal, with an arrest in pollen mitosis (PubMed:17071651). RNAi-mediated knockdown of both RGP1 and RGP2 causes severe developmental defects and strong reduction in total cell wall arabinose (PubMed:21478444). Belongs to the RGP family.