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RI and Bacterial Decoy Proteins1Department of Structural Biology and Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305
Fc
RI, a receptor for IgA-Fc, recruits myeloid cells to attack IgA-coated pathogens. By competing with FcRI for IgA, bacterial decoys, like SSL7 of Staphylococcus aureus, subvert this defense. We examined how pathogen selection has driven the diversification and coevolution of IgA and FcRI. In higher primates, the IgA binding site of FcRI diversified under positive selection, a strong episode occurring in hominoid ancestors about the time of the IgA gene duplication. The differential binding of SSL7 to IgA-Fc of different species correlates with substitution at seven positions in IgA-Fc, two of which were positively selected in higher primates. Two others, which reduce SSL7 binding, emerged during episodes of positive selection in the rabbit and rodent lineages. The FcRI-IgA interaction evolves episodically under two types of positive selection: pressure from pathogen decoys selects for IgA escape variants which, in turn, selects for FcRI variants to keep up with the novel IgA. When FcRI cannot keep up, its function is lost and the gene becomes susceptible to elimination, as occurred in the mouse genome, either by chance or selection on one of the many linked, variable immune system genes. A cluster of positively selected residues presents a putative binding site for unknown IgA-binding factors.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This study was supported by National Institutes of Health Grants AI031168 and AI024258 (to P.P.). P.J.N. was a Lymphoma Research Foundation Fellow.
2 Address correspondence and reprint requests to Dr. Peter Parham, 299 Campus Drive West, Fairchild Building, Stanford University, Stanford, CA 94305. E-mail address: peropa{at}stanford.edu
3 Abbreviations used in this paper: LRC, leukocyte receptor complex; KIR, killer-cell Ig-like receptor; dS, rate of synonymous substitution; dN, rate of nonsynonymous substitution; LRT, likelihood ratio test; NJ, neighbor-joining; SSL7, staphylococcal superantigen-like protein 7.
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