The precursor of beta-lactamase: purification, properties and folding kinetics

The precursor of Escherichia coli RTEM beta-lactamase was purified to homogeneity on a milligram scale by a procedure independent of the binding properties of the protein and refolded to an active, reduced form. For comparing the folding kinetics, the wild-type enzyme was reduced and a mutant was constructed, in which the two cysteines that form a very stable disulfide bond in the RTEM enzyme were both changed into alanines. The rate of folding was determined by directly measuring the increase in enzymatic activity. The reduced precursor folds at least 15 times more slowly than either the reduced mature enzyme or the mature Cys----Ala double mutant under identical conditions. The wild-type enzyme, the Cys----Ala double mutant and the precursor protein all had similar KM values, demonstrating a very similar native state. The slow folding of the precursor compared with the mature form may be an essential and general feature to secure a transport competent conformation necessary for the translocation through a membrane in protein export. This folding assay of a precursor by directly following its enzymatic activity may facilitate the characterization of putative folding modulators in bacterial membrane transport.