Molecular cloning and characterization of the enzyme UDP-glucose: protein transglucosylase from potato

UDP-Glc:protein transglucosylase (UPTG) (EC 2.4.1.112) is an autocatalytic glycosyl-transferase previously postulated as a protein that primes starch biosynthesis. Polyclonal antibodies raised against UPTG purified from potato (Solanum tuberosum L.) tubers were used to screen a potato swelling stolon tip cDNA expression library. The isolation, cloning and sequencing of two cDNAs corresponding to UPTG are described. Recombinant UPTG was labelled after incubation with UDP-[14C]-Glc and Mn2+, indicating that it was enzymatically active. It was determined that purified as well as recombinant UPTG can be reversibly glycosylated by UDP-Glc, UDP-Xyl or UDP-Gal. RNA hybridization studies and western blot analysis indicate that UPTG mRNA and protein are expressed in all potato tissues. Databank searches revealed a high degree of identity between UPTG and several plant sequences that encode for proteins with apparent localization at the cytoplasmic face of the Golgi apparatus and at plasmodesmata. The biochemical properties of UPTG and the apparent lack of a signal peptide that could allow its entrance into plastids argue against the postulated role of UPTG in starch synthesis and point towards a possible role of the protein in the synthesis of cell wall polysaccharides.