Recombinant proteins L and LG. Two new tools for purification of murine antibody fragments

Several bacterial cell wall proteins, like proteins A and G, with valuable affinity for immunoglobulins have been discovered and are currently employed in immunological techniques. In 1988, protein L, a bacterial cell wall protein with Ig-binding capacity, was isolated from the anaerobic bacterial species Peptostreptococcus magnus. Binding data with immunoglobulin fragments suggested that protein L could selectively bind the variable region of human kappa light chains. More recently a recombinant LG fusion protein was expressed in E. coli containing four repeated Ig-binding domains of protein L (fragment B1-4) and two IgG Fc-binding protein G domains (fragment CDC). Recombinant L and LG proteins were tested in the purification of murine monoclonal IgG and their fragments. After affinity-constant evaluation in different buffer systems, high-pressure affinity-chromatography columns were prepared by coupling the proteins to Affi-prep 10 resin and tested with eight different murine monoclonal antibodies and their fragments of various isotypes. Affinity-chromatography experiments confirming radioimmunoassay results showed 75% fragment-binding capacity of protein L and 100% of protein LG. These results evidenced protein LG as the most powerful Ig-binding tool so far described. Therefore, application of these proteins may be suggested in the purification of murine immunoglobulins and their fragments, including the engineered ones.