Four Different Genes Responsible for Nonimmune Immunoglobulin-Binding Activities within a Single Strain ofEscherichia coli

ABSTRACT

Certain Escherichia coli strains bind the Fc fragment of immunoglobulin G (IgG) at the bacterial cell surface. Previous work established that this nonimmune Ig binding depends on several large proteins with apparent molecular masses that can exceed 200 kDa. ForE. coli strain ECOR-9, four distinct genes (designatedeibA, eibC, eibD, andeibE) are responsible for Ig binding. Two eibgenes are linked to eaa genes, which are homologous to genes for the autotransporter family of secreted proteins. With reference to the E. coli K-12 chromosome, theeibA-eaaA cluster is adjacent to trpA (min 28.3) while the eibC-eaaC cluster is adjacent toaspS (min 42.0). Sequence adjacent to theeibA-eaaA cluster converges with that of strain K-12 precisely as observed for the Atlas family of prophages, suggesting that eibA is part of one of these. All four eibgenes, when cloned into plasmid vectors, impart IgG binding to E. coli K-12 strains, and three impart IgA binding also. The IgG binding occurs at the bacterial cell surface, and its expression increases survival in serum by up to 3 orders of magnitude. Theeib sequences predict a C-terminal peptide motif that is characteristic of outer membrane proteins, and the protein sequences show significant similarity near the C terminus to both the YadA virulence factor of Yersinia species and the universal surface protein A II of Moraxella catarrhalis. The sizes predicted for Eib proteins from DNA sequence are much smaller than their apparent sizes on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, possibly reflecting stable oligomerization.