Studies on the binding interaction between peptostreptococcal protein L and a recombinant Fv

Protein L is a multidomain cell wall protein from Peptostreptococcus magnus that is one of a group of proteins capable of binding to antibodies without producing an immune response. In contrast to other immunoglobulin (Ig) binding proteins, such as protein A from Staphylococcus aureus and protein G from group C and G streptococci, which bind at the CH2–CH3 interface, protein L binds exclusively to the VL domain of k-chains. It has previously been shown that a single Ig binding domain of protein L (PpL) has two sites of interaction with the VL domain, with the affinity of the second site up to 50-fold less than that of the first, depending on the nature of the k-chain. The two sites on PpL are distinct, with only one common residue implicated in both interactions. In contrast, the binding sites on the VL domain appear to have 10 residues common to both PpL interactions.The binding interaction between PpL and a 12.6 kDa variable light chain fragment (Fv) has been investigated. Production of a recombinant PpL and recombinant Fv fragment allows the structure of each species and the specific nature of the binding interaction to be examined using a range of techniques, including stopped-flow fluorimetry, circular dichrosism, isothermal titration calorimetry and NMR. To further understand the interaction between PpL and Fv, in particular at the second binding site, mutagenesis of both species has been undertaken and here we discuss the effect of those changes on the affinity of PpL for Fv and vice versa