Recent work in Tadashi Ishi’s laboratory established a new enzyme activity that was never re­ported before, a UDP-arabinopyranose mutase (UAM). The enzyme is capable of converting UDP-Ara pyranose to UDP-Ara furanose (UDP-Ara /) and it was reported to be a reversible reaction (393). This activity was biochemically purified from rice and the corresponding gene was cloned. In rice, two homologous proteins (UAM1; AK098933, UAM2; AK071012) were identified. The recombinant enzyme at thermodynamic equilibrium produces UDP-Ara in a pyranose:furanose ratio of 90:10. The Arabidopsis homologous proteins were initially named as reversibly glycosylated protein, RGP (396, 484). Several isoforms of RGPs exist in the plant kingdom. The Arabidopsis RGP1 (At3g02230) and RGP2 (At5g15650) proteins were found to localize in the Golgi apparatus (396, 483). However, Sagi and coworkers (484) observed that a chimeric RGP-tagged to green fluorescence protein is localized to both Golgi and plasmodesmata. The specific in vivo role of RGP/UAM and the various isoforms in the (i) synthesis of the furanose form of UDP-Ara or as (ii) a reversibly glycosylated protein remains unclear.