Protein G is a multi-domain component of the cell wall of some Streptococcal species and binds to all subclasses of human immunoglobulin G (IgG) by the constant Fc region. The IgG binding function of Streptococcal Protein G is contained in a globular domain of 56 amino acids designated GB. The objectives of this proposal are 1) to determine the tertiary structure of GB and relevant mutants using NMR spectroscopy; 2) to use this domain as a model to study protein folding and stability using NMR, microcalorimetry, CD, fluorescence spectroscopy and site- directed mutagenesis; and 3) to study the basic mechanisms of molecular recognition involved in the high affinity interaction of GB with the heavy-chain, constant region (Fc) of IgG using the above methods. The long term objective is to better understand the function of this class of Fc receptor proteins and engineer new classes of Fc receptor proteins. %%% GB is an ideal protein to probe the relationship of amino acid sequence to structure and stability. It is on the lower limit in size for a stable unique protein structure. Furthermore it achieves its stability without disulfide bonds or tight ligand binding. Its small size permits the application of numerous biophysical techniques to study the folding and stability, in particular NMR methods. Little information is yet available concerning the specific nature of the binding interaction of GB with Fc or the mechanism of its class selectivity. Because Protein G is class specific in its binding to IgG and binds to the portion of the immunoglobulin that defines its class, we believe that our proposed studies will not only help elucidate the mechanism of recognition but also permit the engineering of Fc receptors with new class selectivities. Fc receptors with new properties will have numerous medical and biotechnological applications.
