On the Interaction between Protein L and Immunoglobulins of Various Mammalian Species (1)
Protein L, which is an elongated protein with a molecular mass of about 75 kDa, binds to the framework region of the variable domain of human Ig light chains, preferentially kappa light chains without interfering with the antigen binding site of the antibody.
Most antibodies carrying kappa light chains gave rise to strong signals, whereas the results with lambda light chains were either negative or weakly positive.
Protein L binds exclusively to the gamma region, whereas protein G in addition binds to albumin.
Among these. only Ig light chains gave strong signals with protein L. However. IgG
heavy chains also seemed to interact with the radiolabeled protein L probe in this experiment, which is in contrast to what we have previously experienced in different experimental systems, where heavy chains have been consistently negative in binding experiments with protein L.
13 of 24 species were strongly positive, including primates and rodentia.
Different Ig preparations were then radiolabeled and tested in direct binding experiments with the peptostreptococcus. The large majority of the tested Ig preparations were of the IgG class. More of these Igs reacted with the streptococci and staphylococci, whereas on the other hand the binding of murine monoclonal antibodies to P. magnus bacteria emphasized that protein L binds Ig irrespectively of class and subclass. Not all murine kappa light chains bind to protein L.
Biotinylated protein L reacted strongly with human, baboon, guinea pig, mouse, rat and pig IgG, and weakly with rabbit, horse and goat IgG. Bovine and dog IgG showed no reactivity.
Human, murine and rat IgG were strongly positive whereas rabbit IgG was less reactive. Bovine and chicken IgG did not bind protein L. Clear light chain binding was seen with human, murine, rat and porcine IgG, whereas no interaction was obtained with the bands corresponding to gamma heavy chains. Affinity constants are given in the figure and demonstrate high affinity reactions (> 10^7) for mouse, rat and rabbit polyclonal IgG.
The binding of protein L to human Ig takes place in constant regions of the variable domain of kappa light chains belonging to subgroups I, III and IV, whereas subgroup II shows no affinity. the binding of protein L was highly dependent upon the tertiary structural integrity of the kappa chain VL domain.