INTERACTION BETWEEN STREPTOCOCCAL PROTEIN ARP AND DIFFERENT MOLECULAR-FORMS OF HUMAN IMMUNOGLOBULIN-A

Protein Arp, the IgA-binding protein of the group A Streptococcus, has affinity for the Fc-part of IgA. The binding between protein Arp and several different molecular forms of human IgA was characterized. It w as found that protein Arp bound with higher affinity to uncomplexed fo rms of IgA than to complexed forms (secretory IgA, alpha1-antitrypsin- IgA and alpha1-microglobulin-IgA). Thus, the affinity constant was 2.0 -5.9 x 10(8) M-1 for the binding to monomeric, dimeric, trimeric and q uadrimeric IgA, and 4.5-5.0 x 10(7) M-1 for binding to the complexed f orms. Among the uncomplexed IgA-molecules, the affinity constant was i n the same range for J chain-containing forms (dimeric, trimeric and q uadrimeric IgA) as for forms without J chain (monomeric and a particul ar quadrimeric IgA devoid of J chain). Western blotting demonstrated t hat protein Arp bound exclusively to the alpha-chain of all IgA-forms. Several lines of evidence pointed to a localization of the binding si te to the Calpha3-domain. First, protein Arp did not bind to three N-t erminal alpha-chain fragments which lacked a region corresponding to t he Calpha3-domain, including that from a four-chain myeloma IgA, natur ally occuring in plasma. Second, the binding to dimeric and tri/quadri meric IgA was partially blocked by an added secretory component, which has been suggested to bind to the Calpha2- and Calpha3-domains of the alpha-chain. Finally, alpha1-antitrypsin and alpha1-microglobulin, in the weakly binding IgA-complexes, have been shown to be linked to the Calpha3-domain via the penultimate amino acid residue of the alpha-ch ain peptide, supporting the hypothesis of a localization of the bindin g site of protein Arp to the Calpha3-domain.