Bacterial protein domains with a novel Ig-like fold bind human CEACAM receptors

Abstract

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is the major cause of neonatal sepsis in humans. A critical step to infection is adhesion of bacteria at mucosal surfaces. Though several GBS adhesins have been identified, the host receptor targets of these adhesins remain unknown. We report here that surface-expressed {beta} protein from GBS binds to human CEACAM1 and CEACAM5 receptors. A crystal structure of the complex showed that the IgSF domain in {beta} represents a novel Ig-fold subtype called IgI3, in which unique features allow binding to CEACAM1. Bioinformatic assessments revealed that this newly identified IgI3 fold is not exclusively present in GBS. Instead, the IgI3 fold is predicted to be present in adhesins from other clinically important human pathogens. We confirmed the interaction between CEACAM1 and the predicted IgI3-containing adhesin in two different streptococcal pathogens. Overall, our results indicate that the IgI3 fold could provide a broadly applied mechanism for bacteria to target CEACAMs.