Bacterial proteins that bind to the Fc part of IgG have found widespread use in immunology. A similar protein suitable for the isolation and detection of human IgA has not been described. Here, we show that a 50-residue synthetic peptide, designated streptococcal IgA-binding peptide (Sap) and derived from a streptococcal M protein, can be used for single-step affinity purification of human IgA. High affinity binding of IgA required the presence in Sap of a C-terminal cysteine residue, not present in the intact M protein. Passage of human serum through a Sap column caused depletion of >99% of the IgA, and elution of the column allowed quantitative recovery of highly purified IgA, for which the proportions of the IgA1 and IgA2 subclasses were the same as in whole serum. Moreover, immobilized Sap could be used for single-step purification of secretory IgA of both subclasses from human saliva, with a recovery of ∼45%. The Sap peptide could also be used to specifically detect IgA bound to Ag. Together, these data indicate that Sap is a versatile Fc-binding reagent that may open new possibilities for the characterization of human IgA.
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