(156e) What is Protein A? Natural Variation in IgG-Binding Proteins

Protein A of Staphylococcus aureus is used in most processes for purification of therapeutic monoclonal antibodies. Current commercially-available protein A resins are based primarily on the sequence of protein A from S. aureus pathogen strain 8325-4. There exists an extensive range of natural variation of protein A sequences, and a surprisingly large number of other IgG-binding proteins very distantly related to protein A but differing dramatically from it in sequence. There is no reason to believe that the protein A sequence selected by the pioneers in the field is the optimal choice for bioprocess applications. We tested a range of alternatives and found human IgG-binding activity in proteins with as little as 29 % identity to the sequence of the canonical Z domain of S. aureus protein A. We are conducting biophysical characterization of these binders to elucidate their affinity for human IgG. Based on Isothermal Titration Calorimetry (ITC) analysis, two IgG binders have similar affinity to IgG1 as Protein A Z domain. Surface Plasmon Resonance (SPR) showed that one binder’s association kinetics is as fast as Protein A Z domain.