Barley UDP-glucose pyrophosphorylase (UGPase), a key enzyme for the synthesis of sucrose, cellulose and other saccharides, was expressed in Escherichia coli and purified. Using both native electrophoresis and gel filtration, the recombinant and crude leaf UGPase proteins were found to exist as a mixture of monomers, dimers and higher-order polymers. In order to understand the molecular basis for the oligomerization of UGPase, a conserved Cys residue was replaced (C99S mutant) and several amino acids were substituted (LIV to NIN, KK to LL and LLL to NNN) in a conserved hydrophobic domain (amino acids 117—138). The C99S mutant had about half the Vmax of the wild-type and a 12-fold higher Km for PPi, whereas NIN and LL mutations lowered the Vmax by 12- and 2-fold, respectively, with relatively small effects on substrate Km values (the NNN mutant was insoluble/inactive). The NIN mutation resulted in a low-activity oligomerized enzyme form, with very little monomer formation. Activity staining on native PAGE gels as well as gel-filtration studies demonstrated that the monomer was the sole enzymically active form. Possible implications of the oligomerization status of UGPase for post-translational regulation of the enzyme are discussed.
Abbreviations used: Ni-NTA, Ni2+-nitrilotriacetate; UGPase, UDP-glucose pyrophosphorylase; wt, wild type.
Present address: Finnish Forest Research Institute, Rovaniemi Research Station, P.O. Box 16, 96301 Rovaniemi, Finland.
Present address: Department of Medical Biosciences, Umeå University, 901 87 Umeå, Sweden.