Quaternary structure of assimilatory NADH:nitrate reductase from Chlorella

Gel chromatography experiments over a wide range of protein concentrations showed that Chlorella nitrate reductase is a nonassociating protein with a Stokes radius of 81 A. Sedimentation equilibrium of nitrate reductase in H2O-D2O solvents yielded a partial specific volume of 0.800 +/- 0.014 (n = 12) and a Mr = 360,000 +/- 25,000. No lipid was found associated with nitrate reductase. Cross-linking with the bifunctional reagent, dimethyl suberimidate, and subsequent separation of products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis yielded four protein-staining bands in which the molecular weights of the cross-linked products were integral multiples of the monomeric molecular weight (90,000). Extensive cross-linking of the enzyme resulted in one principal protein-staining band of 360,000, corresponding to a tetramer. The cross-linked tetramer of nitrate reductase appeared to have identical physical properties as the native enzyme. The cross-linking pattern produced by reaction with dimethyl suberimidate suggested that nitrate reductase is an isologous tetramer which has at least two different types of bonding domains. Gel filtration, sedimentation equilibrium, and density gradient experiments at very low enzyme concentrations indicated that nitrate reductase dissociates to a species with a Stokes radius of 54 A, s20.w of 7.1, and Mr = approximately 200,000 at these low enzyme concentrations. No change in specific activity of the enzyme was observed over this concentration range. Treatment of nitrate reductase with trypsin or with cyanogen bromide yielded the number of peptides expected for identical subunits. From these results, it is concluded that Chlorella nitrate reductase is a homotetramer with dihedral symmetry ("dimer of dimers").