The study shows that a protein called SapA helps a common respiratory bug, NTHi, avoid being killed by the human antimicrobial peptide LL‑37. When SapA is missing, the bacteria become much more vulnerable to LL‑37 and other beta‑defensins, but they still resist an alpha‑defensin. SapA works by grabbing onto specific charge‑hydrophobic patterns in these peptides.

Nontypeable <i>Haemophilus influenzae</i> (NTHi) is an opportunistic pathogen associated with respiratory diseases, including otitis media and exacerbations of chronic obstructive pulmonary disease. NTHi exhibits resistance to killing by host antimicrobial peptides (AMPs) mediated by SapA, the substrate binding protein of the <u>s</u>ensitivity to <u>a</u>ntimicrobial <u>p</u>eptides (Sap) transporter. However, the specific mechanisms by which SapA selectively binds various AMPs such as defensins and cathelicidin are unknown. In this study, we report mutational analyses of both defensin AMPs and the SapA binding pocket to define the specificity of AMP recognition. Bactericidal assays revealed that NTHi lacking SapA are more susceptible to human beta defensins and LL-37, while remaining highly resistant to a human alpha defensin. In contrast to homologues, our research underscores the distinct specificity of NTHi SapA, which selectively recognizes and binds to peptides containing the charged-hydrophobic motif PKE and RRY. These findings provide valuable insight into the divergence of SapA among bacterial species and NTHi SapA's ability to selectively interact with specific AMPs to mediate resistance.