Molecular and Structural Basis of Oligopeptide Recognition and Transport by the Ami Transporter System in Pneumococci

ATP-binding cassette (ABC) transport systems are crucial for bacteria to ensure sufficient uptake of nutrients that are not produced de novo or improve the energy balance. The cell surface of the pathobiont Streptococcus pneumoniae (pneumococcus) is decorated with a substantial array of ABC transporters, critically influencing nasopharyngeal colonization and invasive infections. Given the auxotrophic nature of pneumococci for certain amino acids, the Ami ABC transporter system, orchestrating oligopeptide uptake, becomes indispensable in host compartments lacking amino acids. The system comprises five exposed Oligopeptide Binding Proteins (OBPs) and four proteins building the ABC transporter channel. Here, we present a structural analysis of all the OBPs in this system. Multiple crystallographic structures, capturing both open and closed conformations along with complexes involving chemically synthesized peptides, have been solved at high resolution providing insights into the molecular basis of their diverse peptide specificities. Mass spectrometry analysis of oligopeptides demonstrates the remarkable promiscuity of some of these proteins when expressed in Escherichia coli, displaying affinity for a wide range of peptides. Finally, a model is proposed for the complete Ami transport system in complex with its various OBPs. We further disclosed some essential structural changes facilitating oligopeptide transport into the cellular cytoplasm. Thus, the structural analysis of the Ami system provides valuable insights into the mechanism and specificity of oligopeptide binding by the different OBPs, shedding light on the intricacies of the uptake mechanism. Author SummaryThe uptake of diverse oligopeptides enables pneumococcal growth despite auxotrophies and functions as a critical sensor for assessing the composition of the local environment. The identification of additional OBPs in non-encapsulated S. pneumoniae strains suggests their involvement in sensing a broader spectrum of bacterial competitors coexisting with the highly commensal pneumococcus. This study presents a comprehensive analysis of the initial phase of peptide transport mediated by OBPs within the pneumococcal Ami permease system. We disclose a common mechanism for oligopeptide recognition that is modulated in each OBP to accommodate a diverse array of oligopeptides. Understanding how pneumococcus perceives external stimuli and responds to them is imperative for unraveling the transition from a commensal to a pathogenic state.