CHARACTERIZATION OF THE BINDING-PROPERTIES OF PROTEIN LG, AN IMMUNOGLOBULIN-BINDING HYBRID PROTEIN

Protein LG is a 50-kDa hybrid molecule containing four Ig-light-chain- binding domains from protein L of Peptostreptococcus magnus and two Ig G-Fc-binding repeats from streptcoccal protein G. Here we analyse the binding of protein LG to Ig from several mammalian species. Protein LG was shown to bind human IgG of all subclasses and other Ig classes th at carry kappa chains. The binding to human IgG was only marginally in fluenced by changes in temperature (4-37 degrees C) or salt concentrat ion (0-1.6 M), and was stable over a wide pH range (pH 4-10). Protein LG bound to Ig from 11 of 12, mammalian species, including those of ra bbit, mouse and mt. The affinity constants obtained for the interactio ns between protein LG and polyclonal IgG from rabbit (4.0 X 10(9) M(-1 )), mouse (1.7 X 10(9) M(-1)) and rat (1.3 X 10(9) M(-1)) were similar to the value previously reported for the interaction between the hybr id protein and human polyclonal IgG (5.9 X 10(9) M(-1)). The interacti on between protein LG and a mouse IgG mAb was not influenced by the pr esence of the specific protein antigen, nor was the binding of this an tibody to its ligand affected by protein LG. Inhibition experiments de monstrated that the Ig-binding site of one of the fusion partners reta ined its ligand-binding capacity when the other component was occupied . Protein LG selectively absorbed 85-90% of the total Ig present in hu man and rabbit sera and 75-80% of the Ig in sera from mouse and rat. H uman serum depleted of C1q, factor D and properdin and preabsorbed by protein LG could be used as a source for other complement factors. The se data demonstrate that protein LG is a very versatile Ig-binding pro tein.