Plant UDP-Arabinopyranose Mutase

Abstract

Plant cell walls undergo dynamic changes during plant growth and development. Although the cell wall remodeling is an essential feature of plant growth and development, the biosynthesis mechanisms are poorly understood. Arabinan is a pectic polysaccharide which is linked by arabinofuranosyl (Araƒ) residues with α-(1,5) linkage. Arabinofuranosyl residues are a quantifiably important constituent of plant primary and secondary cell walls. Plants use UDP-arabinofuranose (UDP-Araƒ) to synthesize Araƒ regions of the polysaccharides containing Araƒ residues including proteoglycans and glycoproteins. However, it is unknown how UDP-Araƒ is synthesized in plant cells. We succeeded to, for the first time, determine UDP-arabinopyranose mutase (UAM) activity and clone the gene of the enzyme from rice seedlings. UAM catalyzed the interconversion of UDP-arabinopyranose (UDP-Arap) to UDP-Araƒ. Microbial UDP-galactose mutases require reduced FAD for activity, however, the plant UAM do not require the cofactor. Thus, the plant mutase must have different catalytic mechanism. Three proteins were identified from partial amino acid sequence of UAM, which are encoded by Os03g40270, Os04g56520 and Os07g41360. These proteins have more than 80% sequence identity with reversibly glycosylated polypeptide. UAM genes are present in Chlamydomonas, Physcomitrella and pine, suggesting that UAM is widespread in green plants.