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 potat o (Solanum tuberosum L.) tubers were used to screen a potato swelling stole n tip cDNA expression library. The isolation, cloning and sequencing of two cDNAs corresponding to UPTG are described. Recombinant UPTG was labelled a fter incubation with UDP-[C-14]-Glc and Mn2+, indicating that it was enzyma tically active. It was determined that purified as well as recombinant UPTG can be reversibly glycosylated by UDP-Glc, UDP-Xyl or UDP-Gal. RNA hybridi zation studies and western blot analysis indicate that UPTG mRNA and protei n are expressed in all potato tissues. Databank searches revealed a high de gree of identity between UPTG and several plant sequences that encode for p roteins with apparent localization at the cytoplasmic face of the Golgi app aratus and at plasmodesmata. The biochemical properties of UPTG and the app arent lack of a signal peptide that could allow its entrance into plastids argue against the postulated role of UPTG in starch synthesis and point tow ards a possible role of the protein in the synthesis of cell wall polysacch arides. (C) 1999 Editions scientifiques et medicales Elsevier SAS.