Substitution of Proline Residues by 4-Fluoro-l-Proline Affects the Mechanism of the Proline-Rich Antimicrobial Peptide Api137

<b>Background</b>: The well-studied 18-residue-long proline-rich antimicrobial designer peptide Api137 utilizes at least two lethal intracellular mechanisms that target the bacterial 70S ribosome. First, Api137 stalls the ribosome by binding to the peptidyl-transferase center, trapping the release factor, and inhibiting protein expression. Second, Api137 disrupts the assembly of the large 50S subunit of the ribosome, resulting in partially assembled pre-50S dead-end particles that are unable to form the functional 70S ribosome. <b>Methods</b>: All six proline residues in Api137 were substituted with 4<i>S</i>- and 4<i>R</i>-fluoro-l-proline (Fpr), which promote the <i>cis</i>- and <i>trans</i>-conformer ratio of the preceding Xaa-Pro-bond, respectively. The effect on the antibacterial activity was studied using <i>Escherichia coli</i>. The underlying mechanisms were investigated by studying 70S ribosome binding, inhibition of <i>in vitro</i> translation, and ribosome profile analysis. <b>Results</b>: Interestingly, the analogs were equipotent to Api137, except for the 4<i>S</i>-Fpr11 and 4<i>S</i>-Fpr16 analogs, which were four times more or less active, respectively. The most active 4<i>S</i>-Fpr11 analog competed the least with Api137 for its ribosome binding site, suggesting a shifted binding site. Both Fpr14 and the 4<i>S</i>-Fpr16 analogs disturbed 50S subunit assembly less than Api137 or not at all. The strongest effect was observed with the 4<i>R</i>-Fpr16 analog resulting in the lowest 70S ribosome content and the highest pre-50S particle content. This peptide also showed the strongest competition with Api137 for its binding site. However, its antibacterial activity was similar to that of Api137, possibly due to its slower cellular uptake. <b>Conclusions</b>: Api137 inhibits protein translation and disrupts 50S assembly, which can be adjusted by substituting specific proline residues with fluoroproline. 4<i>R</i>-Fpr16 potently inhibits ribosome assembly and offers a novel, unexploited clinical mechanism for future antibiotic development.