DESIGNING OF PEPTIDES WITH IMMUNO-MODULATORY PROPERTIES USING PROTEIN-A AS A PROBE

A series of reports from our laboratory have described the multifariou s properties of protein A of Staphylococcus aureus Cowan I, apart from its Ige binding affinity. Original reports regarding its anti-tumor, anti-toxic, anti-carcinogenic and immunomodulatory properties publishe d earlier by the authors have implicated some uniqueness of this bacte rial protein. It was conceived that such diversified properties must l ie in its specific peptide sequences, rendering it to act and behave a s a multipotent ''Biological Response Modifier'' (BRM). The high resol ution X-ray structure of protein A-Fc complex has been delineated earl ier, and has been the foundation of many protein engineering studies. This structure along with the amino acid sequence data of its four rep etitive domains provided us the basis for designing an octapeptide. Th is octapeptide was synthesized by solid phase peptide synthesis consid ering it as the probable site through which PA binds IgG. This octapep tide (NH2-Gln-Asn-Ala-Phe-Tyr-Glu-Ile-Leu-COOH) is present in the firs t helical segment of B-domain of protein A, and also is a part of doma in D, A and C. This octapeptide has been shown to bind IgG by the immu noblotting technique. The binding affinity of the octapeptide appears to be significantly higher than that of intact protein A, as was revea led by calculation of K-a (association constant) and K-d (dissociation constant) values. This octapeptide might serve as a good immunoadsorb ant for IgG and/or immune complexes.