Immunoglobulin-binding domains:: Protein L from Peptostreptococcus magnus

Protein L is a multidomain cell-wall protein isolated from Peptostreptococcus magnus. it belongs to a group of proteins that contain repeated domains that are able to bind to Igs without stimulating an immune response, the most characterized of this group being Protein A (Staphylococcus aureus) and Protein G (Streptococcus). Both of these proteins bind predominantly to the interface of C(H)2-C(H)3 heavy chains, while Protein L binds exclusively to the V-L domain of the kappa-chain. The function of these proteins in vivo is not clear but it is thought that they enable the bacteria to evade the host's immune system. Two binding sites for kappa-chain on a single Ig-binding domain from Protein L have recently been reported and we give evidence that one site has a 25-55-fold higher affinity for kappa-chain than the second site.